Here I will list the solotop matchups Xin can get.

Tryndamere - Difficulty: Easy.
Explanation: Tryndamere in a little bit squishy on early game and he's got some not-so-good early game because his Ult is what makes he REALLY shine.
How does a Tryndamere helps his team on pre-6?
Well, just get an agressive early game (be careful about the turret, you must dive just if the kill is guaranted) and pay atenttion to the momments where is without his E. Tryndamere can use E to escape and that's very bad. Just engane at him when he haves no E with your full combo. E-Q-Q-Q. And just waste your ignite if his life is extremely low at the end of the combo. (90 or less)

Another tip about ignite: When Tryndamere use his R, pay attention. Use your ignite when its ALMOST ending, do not use when it's over neither when it's on the beginning.

Jax - Difficulty: Hard.

Explanation: Just force him to use his E (and miss it) then engane him then. Preferencially, find out if he has his Q or not 'cause he can use his Q to leap on a friendly minion and escape.

Teemo - Difficulty: Very Hard.

Explanation: Teemo is Xin's #1 counter, cause he's got too much things that harms Xin.

Poke = Teemo is a ranged champion and his Q is ranged as well. Poke is one of the things that harms Xin most.
CC = His blindness on Q is a real pain in the ***, making you miss all you auto-attacks.
Escape = His W boots his speed, which is bad to Xin. Teemo can use this to flee or even to reach Xin if he's a little far.
Outsustain = His E is a poison skill, who totally counters Xin's W passive.

Renekton - Difficulty = Very Hard.

Olaf - Difficulty = Very Easy.

Explanation: Just force him to use his Q, leap on him with your combo. And just engane at an low-HP Olaf if your HP is high, because Olaf's passive gets stronger for each HP he lost.

The more HP he loses, the bigger his attackspeed is.

Pantheon - Difficulty = Extremely Hard.