To get the package openntpd installed was easy, but to get it to work, required some search. In order to have a domU use an independent clock, you have to set a kernel variable. In a running system you do this via the following line:
echo 1 > /proc/sys/xen/independent_wallclock
and to make this setting persistent, add the following line to your /etc/sysctl.conf:
xen.independent_wallclock = 1
Then restart the OpenBSD NTP daemon or install it just after those changes. This will ensure that NTP works properly. Before, I got no complaints from the daemon or in the log files, but the time was off by ~2:10 min … in each and every domU.
// Oliver
PS: all credit goes to Martin at this blog.