I don’t use Xen Server, but the first thing that came up on a search for me was that running a guest OS without Xen Tools (like VMware Tools on ESX guests) was not supported. It appears your VM is using an emulated NIC instead of something native.
According to the Xen Wiki it seems kernels 2.6.26 or later should already have kernel drivers for Xen’s guest OS NICs. If not, can you change the emulated NIC to an Intel E1000? I saw some versions of Xen have that as an option.