As far as I know, Xen does not have a seamless mode.
You didn't mention which guest you want to run seamlessly, but if it's windows, you might be able to get a similar functionality (non-local apps appearing to run locally) by using Windows Terminal Services and RDP.
See http://www.rdesktop.org/ and http://www.cendio.com/seamlessrdp/
If you want to run some other guest, then I believe you have fewer options (none that I'm aware of).