Do you accumulate more XP in Hard or Impossible settings...
 

Because there are more enemy units to fight?

Yes, but each level also requires more xp, so it kinda balances out.

Actually the level requirements are the same, you just get less XP for killing enemy units. But because the troops contain more units in general, it all balances itself out.

Long story short, whether easy or Impossible, you will always get same amount of XP for defeating enemies.

Ok, while I admit that I haven't played hard in long enough to forget whether the xp to level amounts are more than in normal, I know for a FACT that in my friend's easy game, the xp to level amounts are less, and I figured that the same principle would be extended to hard and impossible. I;m going to have to check this, now.