HP labs has performed and published a performance evaluation of OpenVZ and Xen for server consolidation. The 14 pages PDF can be viewed here: HPL-2007-59R1.pdf (418K). Update: links to the paper updated.

I wrote about a Xen/OpenVZ comparison last month here -- the one that was done by a German student as his thesis. I am really glad to see another third party evaluation. (I would never ever trust any comparisons done by or sponsored by vendors.) I also like the level of details provided. Here are quotes from the last section:

For all the configurations and workloads we have tested, Xen incurs higher virtualization overhead than OpenVZ does, resulting in larger difference in application performance when compared to the base Linux case.
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For all the cases tested, the virtualization overhead observed in OpenVZ is limited, and can be neglected in many scenarios.

For all configurations, the Web tier CPU consumption for Xen is roughly twice that of the base system or OpenVZ.

Does that mean OpenVZ is better for scenarios such as Linux servers consolidation? Yes, much better. Does that mean Xen is not good? No, not really. Xen has its applications as well (say when you also want to run Windows on the same piece of hardware), and in fact OpenVZ and Xen can nicely and happily co-exist.