Xen 4.8 on Debian Jessie
For Xen 4.8 RC It seems it's already in Debian Testing https://packages.debian.org/stretch/xen-hypervisor-4.8-amd64
Please use Pinning to install the testing package.
This how to describes the process for Xen 4.4: https://screenzone.eu/howto-xen-4-4-debian-hetzner/
This is also a good simple tutorial http://siddou.tk/2015/08/install-xen-on-debian-8-jessie/
There's also a chapter on Xen in the Debian Admin handbook