Most races have a bonus or minus to experience gained, see the Races page for more information about values.
The XP required to reach level L from level L-1 is given by the ActorLevel.exp_chart function in /tome/load.lua . If L is 29 or below, the XP required to reach it from level L-1 is:
ceil(10 - L(L^2 - 88L + 87)/10)
If L is 30 or above, the XP required is:
ceil(10 - L(L^2 - 173L + 172)/20)
Note that you will not gain any XP from killing enemies that are too far below yourself in level.