Currently, I’m working on setting up Xen 3.0.3 on Debian Etch 64-bit. I installed the debian packages rebooted and used xen-tools to create a Debian etch guest domain. When I used xm create -c debian_etch_guest.cfg vmid=1 I got the following error:

Error: Device 0 (vif) could not be connected. Backend device not found.

I googled for the error and didn’t immediately find a solution to the problem (it may still be there in the search results), but as it turned out, the only thing I needed to do was install bridge-utils:

apt-get install bridge-utils

That’s it, now the guest domain boots without a problem.