Dario,
When you say that using virt-manager is entirely possible with Xen 4.2 on Fedora 18, with which toolstack, XL or XM, do you mean?
I ask because, in my experience with Ubuntu, virt-manager works with the XM toolstack, but NOT with the XL toolstack, under Xen 4.2 on Ubuntu 13.04 and 13.10. See
https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1180281
And although I haven't spent as much time testing it, in my experience, virt-manager doesn't work with the XL toolstack under Xen 4.2 on Fedora 19 beta. I haven't tested virt-manager with Fedora 18.