hey there, building on the recent xen work in nixpkgs - I have created a NixOS template which you can find here:
this is my first stab at creating public nix packages / modules so apologies in advance 
many of the basics are working at this point, details in the README. no RPM yet for aid with install. if you want to try it out before I get that working you should be able to use the nixos installer under HVM mode, and then can switch over to a config using this with PVH.