Is it given according to how much damage a character deals over an adventure or is it given according to the number of kills that character got? I'm just wondering if I need to be trying to give "last hits" to weaker characters.
SirKnight posted a table somewhere. I think it went: Level-difference : xp +2 : 2 +1 : 4 +0 : 10 -1 : 15 -2 : 20 Note that this is by character. So if not all characters in your group have the same level, the lower level ones are going to get more xp for the same adventure until they catch up. What the characters actually do in that adventure doesn't matter, you can have one of them doing nothing and still get full xp if you beat the adventure.
