http://www.networkworld.com/newsletters/linux/2007/0528linux2.html

A new version of the open source Xen virtualization stack was released last week, offering up improved support for 64-bit systems, improved virtual server maintenance tools, and a new API for Xen software partners to play with.

While Xen 3.1 has no earth-shattering new features, some incremental improvements will be useful to serious users of mixed Linux/Windows virtualized server environments.

Xen 3.1 now supports both 32-bit and 64-bit paravirtualized machines with its 64-bit hypervisor. This could allow administrators to consolidate legacy 32-bit Linux and Windows servers and applications onto a Xen machine also hosting new 64-bit servers and software. The 64-bit hypervisor also opens up virtualized machines to a whole new level of memory support, up to 64GB of, vs. the 4GB previously supported.

Check out this article, its talking about Xen 3.1 now supporting 64-bit systems, this is great, the article outlines the benefits this brings, it also mentions live relocation support, which is very useful for maintenance, and in performance/capacity management, being able to move virtual machines around as the situation dictates to manage capacity is an important part of managing the virtual platform.