Due to the fact that I didn't managed in any way to get Xen booting in UEFI mode when asking for help at xen-users, I have been suggested to mail xen-devel to see if someone here can help me with this issue, as this could be bug-related and not a configuration mistake. Original Thread here:
http://lists.xen.org/archives/html/xen-users/2013-12/msg00050.html