You mainly get XP for combat, for exploring and for completing or progressing quests. There's plenty of XP in the game to hit the max level 12 in Act 3 without being a murder hobo (some people complain there's too much!), so the only impact of killing to farm XP is that you might be over-levelled early on and then have a longer period at the end when you're not progressing. I'd personally not want to play that way just for the sake of XP as it would ruin my immersion, but to each their own!
