Well, only some days passed since my latest filesystems comparison. In that article, we see how BTRFS, EXT3, EXT4 and XFS compared each other when used in server and desktop environment.
However, one question remain: what is, performance-wise, the host-side filesystem that you should use for your virtual machine hosting system? In other terms: what host-side filesystem will enable my guest virtual machines to run at full speed? This is not a simple question: while filesystem benchmarking is already a difficult task, it become even more difficult when you take into account the variables added by an HW/OS virtualization layer.