How is Experience handled with Companions?

When I am out in the wilds with a companion, e.g., Aela the Huntress or Mjoll the Lioness, how is it decided who gains how much experience by defeating foes?

Do all the kills my companion achieves get attributed to me, are they splitted up, or does each companion level up on his own?

If I wanted to level up fast, is it better to avoid using companions?

Solution 1:

You do not gain experience from kills in Skyrim. Whenever you use a skill in a meaningful manner, you gain a little bit of experience in that skill. Gain enough experience and that skill levels up, giving you experience towards your next level.

Having a companion may make leveling slower solely through the fact that they take the kills and you have less opportunity to exercise your skills, but their kills do not attribute anything to you at all.