Shattered Kingdoms

Do you gain XP of Petrifying things?

For the casting of the spell, yes. But not in the same sense as damaging things, ie combat exp.

Casting the spells yes, and if it kills I believe you might even get a little for that also.

How about the instakill cleave?

Combat skills you get xp based on combat damage... relative level of NPC, aura difference, etc. See help exp.

If used against a PC, you get no exp.

Petrification is a status change, not a "damaging" spell to my knowledge. You only get the exp for the spell, not a bonus for damage, but you do get exp for both PCs and NPCs.

You used to be able to get decent XP off other PCs, it was a sad day when that changed. I GMed a priest once off duels in fairly short order.

They stopped it to prevent people from power leveling off of a GM by tearing the hell out of it and have a priest continually heal them. It was abused so it got removed.

Wow, Fepel, you receive ten marks out of ten for stating the obvious.

Although I believe it was actually removed because you were never meant to exp off other players in the first place, so it was simply bug removal.