VMware published a report comparing performance of various versions of Xen hypervisor against its ESX server. You can read the report here: http://www.vmware.com/pdf/hypervisor_performance.pdf
Naturally ESX won in the report ( If VMware lost i guess they would not publish it).
In response to this report Simon Crosby, CTO of Xen Source, wrote his opinion defending Xen and questioning some of the benchmark results. It is available on his official blog: http://blogs.xensource.com/simon/?p=12
Xen Source also produced their own report. Unfortunately VMware prohibits publishing results of benchmarks involving VMware products unless they were approved by the company so ESX results were cut out from the paper.
Personally, i like both products. I just hope that both companies will be honest and transparent about the benchmarks so users won't see another "Get the facts" campaign.