Novell LINUX ENTERPRISE SERVER 10 SP2 - VIRTUALIZATION WITH XEN Manual page 20

NOTE: Time Services on Virtual Guests
Hardware clocks commonly are not very precise. All modern operating systems
try to correct the system time compared to the hardware time by means of an
additional time source. To get the correct time on all VM Guest systems, also
activate the network time services on each respective guest or make sure that
the guest uses the system time of the host. For more about Independent
Wallclocks in SUSE Linux Enterprise Server see Section 6.3.4, "Virtual Machine
Clock Settings" (page 67).
For more information about managing virtual machines, see Chapter 4, Managing a
Virtualization Environment (page 27).
14 Virtualization with Xen