Description of problem:
New installation of RHEL5 server.
Once the new kernel boots, the console gets filled with "ISR/XX: Time..."
messages. (Same goes for a serial console)
Might not be related, but the once firstboot finished
(registration/time/date/sound/etc), the machine hanged (still accessible via
SSH/serial console) and gdm refused to start.
The machine exhibit the same problem with FC5-xen kernel.
Version-Release number of selected component (if applicable):
$ rpm -qa | egrep 'kernel|xen|virt' | sort
2 x Opteron 275.
8 x 1GB PC3200/ECC/REG.
1 x On-board LSI Logic Fusion-MPT SCSI controller.
2 x 72GB SCSI. (LVM or Software RAID0)
1 x nVidia Quadro FX 540.
Seems to be related to http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=195
According to the RHEL 5 Release Notes:
Installing the Virtualization feature may cause a time went backwards warning on
HP systems with model numbers xw9300 and xw9400.
To work around this issue for xw9400 machines, configure the BIOS settings to
enable the HPET timer. Note that this option is not available on xw9300 machines.
HP will notify xw9300 and xw9400 users when a new BIOS image is available.
This appears to be covered in the release notes. Please feel free to reopen the
bug if there is any further action required.-
... My mistake.
Yet another victim of the known RTFM disease...
I flashed the BIOS to the latest available version, and NADA.
There's not HPET/"high precision (multimedia) timer" option in the BIOS and I
couldn't find anything about HPET in HP's manual/support information.
More-ever, a simple Google seems to suggest that the 9300 simply does not have
required BIOS support. 
However, the machine -does- boot OK with hpet=disable. (no ISR messages.)
Beyond turning off the HPET support have any adverse effect on Xen?
... And while re-reading your comment:
"Note that this option is not available on xw9300 machines."
/me smack myself on the head with a 2 y/o HP xw9300 machine.
FYI, (for the archive's sake), HP just issued a Xen/RHEL5 HOWTO for the
Kernel 2.6.18-8.1.3.el5xen + latest BIOS seem to work just fine.