With Apache, the performance between the host and virtualized guests was close with Ubuntu 8.04 through Ubuntu 9.04 as the host severely regressed, but with Ubuntu 9.10 and 10.04 LTS the web server performance improved quite a bit while the virtualized performance had not changed much. However, with Ubuntu 10.04 LTS when it was virtualizing a Lucid Lynx installation there was a nice performance spike.
With Ubuntu 8.04 through Ubuntu 9.04, the PostgreSQL performance numbers were close, but with Ubuntu 9.10 there was quite a performance spike due to EXT4 changes and then it regressed back to previous levels with Ubuntu 10.04.
The PostMark disk performance improved each step of the way on the host and when virtualizing the same operating system. Between Ubuntu 9.04 and 9.10 there was quite a performance spike attributed to EXT4 changes within its kernel module, but the older Ubuntu 8.04 LTS release still using EXT3 had stayed the same.
Virtualizing a newer Ubuntu release rather than Ubuntu 8.04 proves to exhibit quite a performance benefit with the more recent releases.