Since 2bits moved into using VPS (Virtual Private Servers) when Xen became available we did not have to directly use other technologies, such as Virtuozzo, OpenVZ (free version of Virtuozzo), or the older User Mode Linux.
However, two recent cases proved what we have suspected for a long time: that indeed Xen is superior to competing technologies when it comes to performance.
In one case, a client contacted us for tuning a slow system. The system gets some 75,000 page views per day on weekdays, and page generation times were 2 seconds or more, and was running on a Virtuozzo host.
2bits cloned the site on a test machine ran timing benchmarks, and found that it peformed faster on the test machine. Then we cut off the external traffic from the live machine using Apache LIMIT directives, and found that page generation times have not improved. Our recommendation to the client was to move to another host. They chose to go for a dedicated server, and that solved their problems.
In a second case, someone posted a request in the forums, where he was getting page generation times of 2+ seconds, while the query components were only 700-800 ms. Upon checking the output of phpinfo, it was apparent that they used OpenVZ. The user was trusting enough to send over a copy of the site's database and files, and we installed it on a Xen VPS.
The results were page execution time of 300 to 600 ms on Xen, as opposed to 2000+ on OpenVZ.
So, the advice is: for Drupal hosting best performance: use Xen if you want a VPS. Avoid Virtuozzo and OpenVZ.
Over the past few years, we have seen more and more clients who complain about poor performance and they ended up being on Virtuozzo. A common theme was the vzfs file system, which virtualizes the host's file system and shares it across the guest instances. This saves a lot of disk space, but from what we have seen, access to the disk is slow. Therefore, database queries are slow, and even serving cached HTML pages from boost is slow. If you are using Virtuozzo, and your site is slow, check to see if you have any of these signs.