The output above is grathered when almost no other services are running except sshd.
While XEN VPS surely consumes more memory than OVZ VPS, I would suggest the Xen VPS can be built in such a way less memory is required to just do the housekeeping.
Of course, that would imply careful provisioning, maybe less "overselling" for the sake of lack of a term. The bottom line is how we can reduce the memory usage, while ensure individual VPS instance can still have fair share of CPU if needed.
Is there anyway in the VPS kernel config which can disable access to certain CPU core, thus reducing the number of kthreads and etc?