tl;dr: Xin is a diver. He has a good gap closer, can isolate his target and kill it pretty fast.
long answer: Melee "Fighter" champions can be in different categories, sorted from least :
- Tank: little damage (or AoE slow DPS), lots of CC/utility. Almost never build damage item. (Sion, Rammus, Amumu, ...)
- Juggernaut: high damage, very little mobility/CC. Usually build 1-2 damage items. (Garen, Darius, Nasus, ...)
- Brawlers/Bruisers: average in all respects. The default category for those that don't really fit elsewhere, with too much utility/CC to be Juggernaut, but not enough potential to kill a priority target to be Diver. (Trundle, Volibear, Olaf, Udyr, maybe Shyvana)
- Diver: good mobility, at least to close the gap if not to escape, and good target selection. Medium-to-high damage. Basically a toned-down assassin, a bit less bursty, but more tanky. Usually build 2-3 damage items to be able to kill their target, and the rest is tankyness to survive after diving. (Irelia, Vi, Aatrox, Wukong...)
- Carry: build mostly damage and rely on killing their enemy first/outplaying him to survive (Fiora, Yasuo, Yi, Tryndamere, Riven, ...)
Technically, most Assassins are melee too, and there can be some overlap between Divers/Carries and Assassins. But Assassins are generally considered their own category, and not part of the Fighter group.
There are champs who don't fit these categories exactly, of course. For instance, Mundo builds like a Tank/Juggernaut, but he has almost no CC/utility (so not a Tank) and a very powerful ranged poke (so not a Juggernaut). Renekton has a lot of mobility, a single target stun and a bit of burst (could be a Diver) but also good tankyness and AoE damage (more typical of Bruisers).