Since 24.11, the Xen Project now has its own functioning options, available under the virtualisation.xen.* option set.

There seems to be a further history of this option set down the history lane of that other repo:

commit 5c1f8cbc70cd5e6867ef6a2a06d27a40daa07010
Author: Eelco Dolstra <eelco.dolstra@logicblox.com>
Date:   Thu Oct 10 13:28:20 2013 +0200

    Move all of NixOS to nixos/ in preparation of the repository merge

diff --git a/nixos/modules/virtualisation/xen-dom0.nix b/nixos/modules/virtualisation/xen-dom0.nix