While I haven't yet tried this myself, Xen 4.0.1 supports :"Remus Fault Tolerance: Live transactional synchronization of VM state between physical servers. run guests synchronized on multiple hosts simultaneously for preventing downtime from hardware failures. Remus support in Xen 4.0.0 requires linux-2.6.18-xen dom0 kernel. Pvops dom0 kernel (2.6.32) support for Remus is available in Xen 4.0.1. For more information see Remus wiki page"
Note that scalability and redundancy aren't necessarily related. Usually you get either redundancy (i.e. RAID1) or scalability (i.e. RAID0)
More info on Remus is available on http://wiki.xen.org/xenwiki/Remus
If you don't require quite as high redundancy as Remus (apparently) can provide, Xen also provides "ordinary" live migration og VMs between different Xen-domains.
Either way, one single Xen hypervisor won't do what you're asking for. Every server must run it's own hypervisor, but they can be configured to provide RAID1-type redundancy.