Party members killing each other is something that could reasonably happen in an evil party, or because of some other great event (eg. there's a card in the Deck of Many Cards that makes one of your friends turn against you). It's actually a very interesting thing to happen, when it occurss through normal playing out of the story; I'm playing in an evil campaign now, and it's quite common for party members to threaten violence at each other, it'll come to blows sooner or later, and I think it will be fun when it does.
However, it sounds as if your players are just abusing the game mechanics. This is out-of-character metagaming of the worst kind.
You have to remember something very important about the nature of XP though, you get them for overcoming challenges; if a party member allows himself to be killed, then there's no challenge involved, so no XP. It one of the situations mentioned above, if both characters fought to the best of their abilities, and it was in character that they be fighting each other, then XP should be awarded, as is appropriate for an encounter of the challenge rating of the PC that was killed.
How is this viable as a method of getting XP? All resurrection spells have very high component costs involved, and all but the best (true resurrection) carry negative consequences, including lost levels etc, for the character killed.
Still, the two important things here are the roleplaying and the challenge. Both need to be there for XP to be awarded. If you were feeling cruel and wanted to punish your PCs for abusing game mechanics, you could allow them to kill one of the party members and then not award XP, and impose a harsh penalty for the character's resurrection afterwards.
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