>

Nothing. China is in that weird spot no matter what action they choose, it will do poorly.
1) If China support Russia--well, possible sanctions, possible nothing. West opinion of China will not improve.
2) If China support Ukraine-- No additional benefits, hostile Russia on their northern border, western opinion will not improve--and certainly wouldn't help get Taiwan back. if Russia collapses/fall into a western controlled puppet (Yeltsin II), it wouldn't help China either. 
So basically geopolitics dictate China to sit on the fence. No matter who wins in this war, it is unlikely they can cut China out from the future. 
So the answer is to do nothing.
I can't see Xi stepping into Ukraine. >

what would happen if Xi goes to Kyiv to show his public support for Zelensky? >

That would be counter to what he has been doing: increasing support for Russia.  source >

then what would be the appropriate way for Xi to respond to Zelensky's invitation?