In order to effectively use an NTP source on a Xen Virtual Machine the sysctl kernel parameter xen.independent_wallclock must be set to 1.
Local clock on a Xen Linux machine is by default in sync with the clock of the Physical Host (Dom0), in order to change it and use time syncronization services as NTP, a kernel parameter must be set on the virtual machine:
echo 1 > /proc/sys/xen/independent_wallclock
For a boot resilitent configuration in the /etc/sysctl.conf
of the virtual host add:
# Set independent wall clock time
xen.independent_wallclock=1