How to install Opensolaris on XEN ?
For the real die-hard admins (as you have to BFU your installation), there is an Howto for installing OpenSolaris on Xen.
By the way: There is an really good explanation in of the differences and positioning of Solaris Zones and Xen Domains in the FAQ section of the Opensolaris Xen-Community:
The different approaches possess efficiency, observability, sharing, utilization, isolation and manageability attributes which can be markedly different. The Xen approach is good for isolation, though presents the need to manage several complete operating environments, which can be a feature or a problem, depending on what you're trying to do! In contrast, because of the high degree of resource sharing that is possible, Zones are good for utilization, observability, and manageability.