With Ubuntu 8.10 it not possilble anymore to run Ubuntu 8.10 as Dom0 host. Ubuntu does not supply a suitable Kernel for this purpose. Of course it is possible to compile your own Kernel using the XEN patches from xen.org
It's still possible to run Ubuntu 8.10 as a DomU guest OS. Ubuntu advices to use KVM for virtualization. This requires CPU's with virtualization extensions (hardware virtualization).
For now, Ubuntu 8.04LTS is the last version to include a XEN-enabled Kernel.