Here's my attempt that going in depth into the mastery trees. I'll try to give my opinion on each and every option, and tell how it relates positively, or negatively to Xin Zhao.
Here are some quick summaries of the trees.
Xin Zhao can benefit greatly from the offensive tree. However, we find ourselves not needing the benefits of the offense tree when building in the jungle. From how we build we don't get many benefits from the tree. On top of that, the armor penetration you would normally go for is met short as your passive will be you enough penetration for what you want.
The defense tree is everything we would want and more as a jungler. Between the flat HP, hp per level and resistances we find ourselves starting off tanky and growing tankier throughout the rest of the game. We also find ourselves getting extra scaling and tenacity as well as reduced damage in the jungle. All of this and more make this the best tree for you to grab as a jungler.
Here's a tree we find ourselves tackling that may be avoided by junglers hoping to grab % penetration in the offense tree. Since Xin makes full use of his passive, he has less need for the penetration in jungle, as his mark will always be on the big jungle creep. Also, since he's maxing his E first in the jungle, a lot of the damage he deals will be magic damage.
So let's see how we benefit from the utility tree. For one we lose cooldown off of flash, we drop the cooldown of flash by 45 seconds, which helps you have it up to get more offensive (or even defense) opportunities. We also gain a fair bit of movement speed to speed up our jungle and help with ganks. At the end of the tree we also gain an extra 30 seconds on the red and blue buffs, which Xin makes excellent use of both.
From here we shall tackle each mastery in every tree, this is really only for people who REALLY want to read into masteries. Remember a page isn't perfect and you can always build to fit your own style or situation.
This is a very solid mastery if you were to use the summoners accociated with it. For information as to which summoners you should use, head over to that section of the guide.
Exhaust: Reduces the armor and MR of the foe you use this on by 10. This is great for early game ganking as it drops their resistances to help you do practically true damage.
Ignite: AD and AP is gained while the summoner is on AD. It helps finish a kill in lane, but won't help much as a jungler.
Ghost: Increases movement speed when you use ghost by a great amount. Worth using if you take ghost.
Garrison: You're not a bot lane character in dominion, so you wouldn't want this.
This mastery gives you attack speed which can help clear the jungle since Machete deals extra true damage on each hit.
4% CDR which actually is quite useful to Xin Zhao. All the items you build that involve CDR hit 10%, so hitting 40% would force Xin to build 4 full items, in the jungle that's difficult, so taking these masteries and running CDR runes isn't a bad idea.
This applies bonus damage to minions and neutral monsters that you auto attack. We skip this because of the damage we get through our E and how fast we get madred's razor.
AD per level and opens up armor penetration down the tree. If you want to grab the % penetration this is required.
Xin doesn't need AP, so this is ignorable.
Increases damage to turrets by 5%, it's really not a mastery worth it on a jungler. If you were to push down the tower as a melee champion, this mastery wouldn't make a big difference.
This mastery increases your damage by up to 2%, it's really such a small amount that it's negligible. There are a better use of 3 points than to get this mastery on a tank.
If you need armor penetration this is a fine grab on Xin Zhao.
Magic penetration isn't a stat Xin Zhao really needs, sure he deals a bit of magic damage but where it counts there shouldn't be much magic resistance, and 10% reduction isn't going to help enough.
Critical damage? It's too rare considering the tank build we're using.
This is a fine grab on a top Xin Zhao, but as the jungler you aren't going to make the most use of this mastery, nor will you be building towards AD.
Flat ability power? We aren't going to use this stat.
You don't build AP, so this isn't worth it.
You don't get critical hits, so this mastery isn't worth investing in.
Flat armor penetration is a solid stat on Xin Zhao, however going this deep in the offense tree means that you're losing our in the defense tree.
You don't build AP, so this mastery isn't used.
While early you would make good use of this mastery you don't really use it later in the game, ultimately, you're way too far deep into the offense tree if you're here as a jungler.
This is a very good summoner if you use the summoner accociated with the mastery. As a jungle Xin Zhao we would definitely take this to get the extra gold for the smite mastery, since you'll be taking smite no matter what.
Cleanse: Increases the disable reduction by a second, it's not anything you would be carrying on a tanky champion.
Heal: Passively increasing health by 5 per level, again this isn't a very useful summoner for a jungle champion.
Smite: Extra gold on a jungler? Yes! This adds a lot of gold and should be grabbed on a jungler.
Barrier: 20 extra shield amount is good for an early usage, but really seems like a lackluster point compared to the rest. This isn't a useful summoner on a tanky champion.
Between the health pots and the regeneration in the W, Xin will be getting more than enough health regen.
Health per level is a very good stat to get on a champion like Xin. The health is going to build up overtime for free, and as a tank what more could you want? Oh you have to level this to get Veteran Scars? Icing on the cake, we'll talk about that mastery later.
Reduces damage taken from monsters. VERY good for a jungler, and it opens up Bladed Armor, which is a very VERY good mastery for junglers, but we'll talk about that in a bit.
Armor? Armor! This is a helpful mastery for a jungler, however we only put two points in it because of all the points we put into the first row.
Magic resistance is a helpful statistic for tanks, however as a jungler, we'll be dealing with armor for the most part, the APs will stick to mid lane so we have time to build our own MR, we don't need our masteries to give it to us.
Deals damage to monsters that hurt you, as a tanky champion who built for durability, this is going to help you clear quite a bit. This is a very solid mastery to pick up.
As a champion who deals mostly with neutral monsters, reducing champion damage isn't going to help you much. You don't do much trading like lane champions, and if a champion were to go all-in you most likely wouldn't be standing with 2 health. While in theory it COULD help you, it's such a long shot that it's not worth investing in this mastery or it's higher point.
Reduces slows by 15%, this helps a lot since we will be building as a tank and will be initiating for our team. This helps us get less kited in fights.
30 flat HP for free? This is one of my favorite masteries, so my biased says to always get it. It's a great mastery to give you some good health right from the start of the game.
5% damage from turrets, while it sounds nice, it's so niche and random that you'd be better off investing in anything else. It's really just something that you don't need to invest mastery points into, but while your buy more items, you will be able to tank turret shots better overall.
Reducing damage from champion basic attacks is just like unyielding, it's useful, but not useful enough to merit the use of your mastery points. It's something you'll build into.
Reduces the duration of CC. Vital to all tanks, a VERY solid mastery. It's not something you can just build more than one of, so all the extra points help with your task to becoming the tank of your team!
4% maximum health increase. You'll find yourself building a lot of flat HP, so letting it scale up by 4% in quite useful.
Getting points based on the number of opponents is not the beset mastery. But what you have to realize is that out of the points available it's going to offer you stats that will be useful when they kick in. Say you're counterganking, well you're going to get some magic damage tops or jungles, and some physical damage. Well this is going to cover both of those instances, instead of dumping the points into just single armor or magic resist. It's also a great mastery for invasions, as the early extra armor and MR may save you from a bit of early damage.
5% on bonus armor and resistances just isn't enough to dump 3 defense points so late, there are much better masteries to put points in. Even with 200 EXTRA armor you're only getting an extra 10 armor. At this point 10 armor makes such a little difference that it just shows how worthless this mastery is.
10% less time spent dead really doesn't mean much to you. You don't want to die, dying is bad. You don't want to waste mastery points for lessening a bad thing. Just don't do it!
10% less damage from critical strikes isn't really worth it. It's such a small portion of the damage you'd take, that it isn't worth such a late point in your mastery tree. Remember, the majority of your mastery points are there to help your early, not necessarily your late. (There are exceptions, but this isn't one).
3% reduced damage taken from EVERYTHING? Hell yes. As a tank 3% less damage from everything is just too good to pass up.
Revive: Grants bonus health for reviving, but since you wouldn't take revive on Xin, this is a very neglectable benefit.
Teleport: Reduces cast time by 0.5 seconds. Doesn't really help much even if we ran teleport.
Flash: -15 CDR, solid for flash reduction.
Clarity: 25% extra mana, Xin shouldn't have mana problems, and even if he did, this summoner and mastery is not the answer.
Clairvoyance: Reveals champions seen by clairvoyance, we keep this summoner to the support, there's no reason to take this mastery for clairvoyance on your jungler.
Movement speed when leaving the jungle to get in place for ganks. Also helps you get to camps faster. Very solid mastery.
Mana regen to help you get more mana, while it's a solid mastery it's not worth losing the movement speed for it, since you should be able to manage your mana more efficiently.
Helps you recall a second earlier in Summoner's Rift. It's an alright mastery that's usually grabbed for filler. It's a fine choice to drop your extra point in.
Extra vision when wards are placed for the first 5 seconds. It's a point not really found to be that useful. As a tanky jungler you're expected to be like the second ward placer on the team, but this is still not worth the point used.
Lower cooldown on Flash and Smite? It's a very solid mastery to go into if you find yourself going down the utility tree. 10% cooldown reduction on summoners is cool as not much really lowers the cooldown on summoner spells.
Mana per level, while it isn't a bad stat, it cannot compare to the other options in the row, it's just not something you really need on Xin. There are better options.
Reduces the cooldown on activation items, since you find yourself with a good bit of these it's a solid choice to drop an extra point into.
Extra gold over time is an interesting stat on a jungler. You should still be able to clear while having this statistic, but you're really dipping too far into the utility tree that you're taking too much out of the defense tree. It's an alright grab though.
When you go for 9 points in the utility tree, this is the point to get. Increasing the buff duration is a very useful thing, and the only thing that can do it is this mastery. It's a very solid point to get, and as a jungler you utilize this to more than the most on your team.
Lifesteal and spellvamp, when you have the W sustain Xin's skill gives, you really don't need a small amount of healing off of skills and auto attacks.
You do not need the biscuit as you have your passive and many health pots at the start of the game. It's really just a waste of a mastery point this far into the tree when you already have so many better options.
Extra starting gold gives you another potion, but with your W passive you really don't need it, there are other places to put your mastery points than in this mastery.
This increases your experience gain by up to 5%, this is a great grab for if you are heading this deep into the utility tree. While it's recommended to not going this far, if you did this would be the mastery in this section of the tree to grab as getting your ultimate opens up a lot of possibilities.
Strength of Spirit
Extra health regen for the amount of mana you build, outside of frozen hear we don't really build much mana, so this really isn't a mastery you should even consider dropping points into.
A 60 second ward that you get at the start of the game. It's really something you don't need to consider as there are far too many options this far in for Xin that you wouldn't even begin to consider Explorer, besides you wouldn't even want to grab Biscuiteer, the prereq to get this mastery.
5 gold for auto attacks you land on enemy champions based on a 5 second cooldown. Sadly you don't see many enemy champions in the jungle, so this is a heavily wasted mastery on a jungler.
If you were to go this far into the utility tree, may you be happy to find 6% CDR close to the end. While it's not the most important thing, as Xin can build many items efficiently that give CDR, it's a nice grab if you're shooting very far for the utility tree.
Last, but not least, Nimble. Extra movements speed is a godsend on a jungler. While it's not something to go that far into the tree for as you are wasting a lot of durability building so far into the utility tree. If you made it to this point at least you can say you walked away with a nice boost of extra movement speed. It's a hard stat to build outside of boots, and Xin can't get enough of it as it only helps allow you to get into position to use all of your skills to be the best disrupting tank as possible.