In the hope that in particular FreeBSD and NetBSD will work well under virtualisation, I try to keep track of the current status.
| Xen 4.2 + NetBSD 6.1.2 | Debian 7.2 kvm | |
|---|---|---|
| FreeBSD 8.4 x86-32 | OK | OK |
| FreeBSD 8.4 x86-64 | OK | OK |
| FreeBSD 9.2 x86-32 | OK | OK |
| FreeBSD 9.2 x86-64 | OK | OK |
| FreeBSD 10.0 x86-32 | filesystem malfunctions + panics at first incoming ssh | filesystem malfunctions |
| FreeBSD 10.0 x86-64 | panics at first incoming ssh | OK |
| NetBSD 6.0.3 x86-32 | OK | OK |
| NetBSD 6.0.3 x86-64 | OK | OK |
| NetBSD 6.1.2 x86-32 | OK | OK |
| NetBSD 6.1.2 x86-64 | OK | OK |
| OpenBSD 5.3, 5.4 x86-32 | unstable, extremely slow | extremely slow |
| OpenBSD 5.3, 5.4 x86-64 | unstable, extremely slow | OK |
| DragonflyBSD 3.4 x86-32 | freezes within hours under load | somewhat unstable |
| DragonflyBSD 3.4,3.6,3.8 x86-64 | freezes or panics within hours | very unstable |
NetBSD is the only BSD flavour that works in all cases. Previous FreeBSD releases also worked, but FreeBSD 10 is a sad exception. I tried to get the attention of the FreeBSD kernel people early in the FreeBSD 10 release cycle, but I wasn't very successful. (See e.g., the ssh bug report.) I never bothered to report the problem with 32-bit installs under both KVM and Xen; here the filesystem doesn't work enough to allow a single ports build.
OpenBSD probably works poorly by design, since its lead developer despises virtualisation. Not sure about DragonflyBSD. Since nothing works for these projects, I assume it is expected, and that bug reports would just annoy their maintainers.