Resource efficient dist with support for Xen.
 

I'm quite new to Linux and is looking for a distribution that doesn't use that much resources. The only thing i'm going to install in it is Xen. It's going to host 2-3 virtual machines.

I'm going to install it in a small HP server with a dual core XEON cpu.

As I'm quite new, a simple dist is a plus. I'm not that much for hacking loads of config files to get everything running.

All distros will support Xen.
Anything very lean will be disqualified by "I'm not that much for hacking loads of config files to get everything running."

So I'm leaning towards Debian or CentOS for you - don't install the whole thing in either case: you just want a minimal server and Xen. These should handle your mobo/cpu without fuss, allow managed updates, and the minimal install should be light enough to keep you happy.