I would like to bring this up one more time. Although installing ZeroShell under XenServer works as described in the howto it has considerable draw backs because it then runs as a HVM guest.
In order to gain benefit of the full power Xen overs ZeroShell should be run as a PV guest and therefore it would need a PV-enabled kernel as well as PV drivers.
From my honest perspective this is something that really should be considered to become part of the product. Maybe providing a dedicated kernel as the one for 64bit environments? We’ve got the century of NaaS (Network as a Service) and being able to run machines and well known network related hardware (switches, routers, …) as virtualized appliances has gone far behind just being an idea but more and more what’s being done out there.