Скорее всего из-за механизма виртуализации.
XEN
* Linux, paravirtualization integrated in 2.6.23, patches for other versions exist
* Minix
* Plan 9 from Bell Labs
* NetBSD (NetBSD 2.0 has support for Xen 1.2, NetBSD 3.0 has support for Xen 2.0, and NetBSD 3.1 supports Xen 3.0)
* OpenBSD, announced here but discontinued
* FreeBSD (Limited, experimental support for Xen 3 in 8-CURRENT [1])
* FreeBSD 5.3 currently broken? Actively being worked on
* FreeBSD 7-CURRENT can be patched; works.
* OpenSolaris (See The Xen Community On OpenSolaris)
* NetWare (at Brainshare 2005, Novell showed a port that can run as a Xen guest)
* GNU/Hurd/Mach (gnumach-1-branch-Xen-branch)
* OZONE (has support for Xen v1.2)
http://wiki.xensource.com/xenwiki/OSCompatibility
http://en.wikipedia.org/wiki/Xen
Virtuozzo
* CentOS 3.x, 4.x, 5.x
* Debian 3.0, 3.1, 4.0
* Fedora Core 1, 2, 3, 4, 5, 6, 7, 8 ,9
* Red Hat 7.1, 7.3, 9
* Red Hat Enterprise 3, 4, 5
* SUSE 8.2, 9.0, 9.1, 9.2, 9.3, 10, 10.1, 10.2, 10.3
* SUSE Enterprise Server 9, 10
* Ubuntu 5.10, 6.06, 6.10, 7.04, 7.10, 8.04, 8.10
http://www.parallels.com/en/products/virtuozzo/specs/linux