I know this is going to be an inaccurate figure, but it seems that if you're good aligned, killing evil mobs, you're going to get roughly twice the experience that an equal level neutral character might. This is of course taking into account that you have a group of three. The bonus for killing evil mobs as a good aligned character is a little bit less when you're by yourself.

I've always said that it's harder to level up a wood-elf bard or cloud giant ranger than it is to level up an elf paladin, speaking only on conditions of the xp penalty, and not necessarily inherent class abilities.

Basically: If you're killing things of the opposite alignment, you get a very noticeable difference in XP gains.

And if you're an Orc leveling up with two others Orcs... Whoah, watch out. They get a monster bonus to XP gains.