So, I'm jungling Olaf, and I see a xin alone at half health who just stole my red buff. Being at full health, a attack, not knowing that he killed me within 2 seconds. How is it possible to beat a champ that you can't touch?

Exhaust, Frozen Heart, Randuin's, Armor, Malphite...

Need I go on?

I dont know how you died that fast, was he feed? what level were you? you have an ult where you take no cc, life steal with w and hopefully wriggles. w spam and true dmg from e. either you did something wrong or you need to just learn from your mistakes.
tekno- right on point MOAR ARMOR.

use ult at start of fight for armor buff. Normally you don't want to 1v1 xin regardless of whether he's fed or not

Xin is indeed a beast to 1v1 even someone as tanky dps as Shyvana should be wary when 1v1 against Xin in the jungle. His kickup (three talon strike) thankfully no longer scales on critical strikes but it still hurts and is a very tough CC without backup. Though Olaf should have had the upper hand with his ultimate, best advice is to grab a friend and practice 1v1 Xin vs your usual champ so you can better learn him before you jump on a Xin in ranked. (That is how I learned to not 1v1 Lee Sin in most situations)

Jax can beat Xin, so can Lee Sin and Udyr.

I use rengar, jump on him, boom...dead.

jax master flex

xin passive is way to strong really, if you saw him in your jungle that should aleard mid top bot to gank and have a 2v1, i never try to solo a jungler piriod, never now there skills or runs cause its not ur lane partner

His "e" makes him heal which is kinda op in a way so you probably ignite him so he doesn't heal as much. Then stun him, slow him then kill him. simple.