is it possible to deploy squeeze domU under Lenny dom0 ? using xen-tools and debootrap for example any big problems if possible??
I can't think of anything that would stop this working.
Your limitation would be trying to run 64bit installs under a 32bit host. But yeah, you can run any distribution under Xen as paravirtualised provided there is a kernel available. With hypervirtualisation you can run any kernel regardless.
As an example, my Opensuse Xen server has run various versions of ubuntu, arch and LMDE all paravirtualised, and a mix of 32bit and 64bit (the host is 64bit). The kernel versions of all of these are later than the host.
deboostrap sometimes has problems with a +1 install. However, if you have another source of the squeeze VM contents (a straight install using even non-KVM qemu on a laptop would work), then you are set. You may find that not every kernel is in the right executable format for lenny dom0 to load.