OT: Xen

a message of 14 lines which said:

Fairly well -- a lot better than (eg) vservers, and almost certainly
better than UMLs.

Because they are different virtualisation solutions with different
requirments. If you have unrelated customers, who do not trust each
other, Xen (or UML) is OK. If you just want to put one service on a
different machine but do not have the money (or the rack space) to
dedicate a box to just DHCP, Linux Vservers or FreeBSD jails are fine.