Experience Points

Experience points (short XP) are a measurement for how your character is progressing. The player character can earn experience points for killing creatures and solving quests. They are used to determine when a character should gain a new level. Different amounts of experience points are awarded for killing more dangerous creatures. For example, in Fallout 3, killing a deathclaw will get you 50 experience points while killing a radroach will give you 1 experience point. Experience points can also be gained for disarming mines and traps, picking locks, hacking computers, passing speech challenges and occasionally doing sections of a quest.

Fallout 3 difficulty modifier
Changing the difficulty level will also change experience gained from killing enemies. On Very Hard you will receive 150% of the regular experience for a kill, while a kill on Very Easy will only yield 50%. For example, a Deathclaw is worth 25 XP on Very Easy, 50XP on Normal and 75XP on Very Hard.

This can be a very useful way of earning XP. Get a good weapon and some ammo (preferably a hunting rifle or laser rifle in condition above 75% ish) and discover Old Olney. Then proceed to kill all the deathclaws in the area (7 above ground 7 below. Check the link Old Olney for more information on Old Olney) playing on very easy or easy depending on ho powerful your gun is then switch to very hard when the deathclaw is close to dying and finish it off granting you up to 83% XP if well rested and even higher with perks. Another advantage to this method is that the deathclaws above and below ground repawn every 73 hours so when you have cleared the area head back to your house re-stock repair and rest for 3 days and head back. While this means that you can gain up to three times as much experience from fights on Very Hard compared to Very Easy, this is somewhat compensated by the enemies being much tougher. In the end, it depends on the player´s skills and play style which difficulty setting allows for faster leveling.

However, the amount of XP earned for dispatching a single enemy is capped at 108; this can happen frequently on the Very Hard difficulty after killing powerful enemies such as Enclave Soldiers and Feral Ghoul Reavers. Although this was updated/patched and the player now receives the right amount of XP for tougher kills.Similarly, on Very Easy and Easy, weak enemies like the Radroach and Bloatfly are worth no XP at all.