Diablo 3 Experience Points Explained

In Diablo 3, you get drastically diminished experience point gains from killing monsters lower than yourself, and you also get increased experience points for killing monsters higher level than yourself. Below is a chart showing exactly how much XP you gain or lose.

Monster level compared to character level XP gain
3 levels higher and above 125%
2 levels higher 120%
1 level higher 115%
Same level monster 100%
1 level lower 90%
2 levels lower 80%
3 levels lower 70%
4 levels lower 60%
5 levels lower 45%
6 levels lower 30% (no +XP/kill from gear)
7 levels lower 15% (no +XP/kill from gear)
8 levels lower 5% (no +XP/kill from gear)
9 levels lower 1% (no +XP/kill from gear)

Also here is an XP image chart by Malloy626:

XP Chart Click for the Full Size

A chart that displays each act and difficulty. The yellow portion is what I'm guessing is Blizzard's ideal leveling curve.

Note that at 30% or less, +exp and +%exp no longer functions.

Expanded explanation;

This chart provides you with how much experience your character will receive when killing monsters in certain areas. The "Min", "Mid", and "Max" columns represent lowest, average (rounded), and highest monster levels in the act. So if you are level 50 and kill a level 50 monster, you get 100% of the exp, change your level to 60 when killing the level 50 monster and you get 0% of the exp.
  • char level +10 or more of monster level = 0%
  • +9 = 1% (+exp effects nullified)
  • +8 = 5% (+exp effects nullified)
  • +7 = 15% (+exp effects nullified)
  • +6 = 30% (+exp effects nullified)
  • +5 = 45%
  • +4 = 60%
  • +3 = 70%
  • +2 = 80%
  • +1 = 90%
  • +0 = 100%
  • -1 = 115%
  • -2 = 120%
  • -3 or less = 125%