When Kha'zix levels up his ultimate he get to evolve one of he's special abilities
What i recommend is that in the first point you evolve you Void spike(W), Second time wich is in lvl 11 i recommend that you evolve you LEAP (E) for the longer range and reseted coldown when Kha gets a kill or assist
and the lastpoint wich you get in lvl 16 when you spec you last point in a ability its up to you if you want the last one in you TASTE THEIR FEAR (Q) or VOID ASSAULT (R) It's all up to you

always try to use your passive *Unseen Threat* for the slow so your team mate who's ganking you
so they can come up and nuke your enemy's Health,

Farming is easy when you hit lvl 6 with Kha'zix you just use VOID SPIKE (W) with UNSEEN THREAT and you pretty much 1 shot all the minions in the wave,
but sometimes it takes a while to get to lvl 6 and spec that point in ult to evolve the Void spike(W)but i just farm like every one else just trying to go for as many last hits as possible and still trying to zone out the enemy so he/she dont get any gold from the farm, my tip is to use the bushes in lvl 3 to jump on the enemy do you Void spike and an auto attack then your q to sink your enemy HP

Always try to keep your passive up at all times

keep using you Void spike(W) to both farm and to keep your enemy laner away from the Cs and Xp

If you se un opportunity to use your TASTE THEIR FEAR (Q) when there is no other minions or champions near your target for the extra damage wich in some cases might save your life