Xen supported virtualization types
Xen supports running two different
types of guests. Xen guests are often
called as domUs (unprivileged
domains). Both guest types (PV, HVM)
can be used at the same time on a
single Xen system.
Xen Paravirtualization (PV)
Paravirtualization is an efficient and
lightweight virtualization technique
introduced by Xen, later adopted also
by other virtualization solutions.
Paravirtualization doesn't require
virtualization extensions from the
host CPU. However paravirtualized
guests require special kernel that is
ported to run natively on Xen, so the
guests are aware of the hypervisor and
can run efficiently without emulation
or virtual emulated hardware. Xen PV
guest kernels exist for Linux, NetBSD,
FreeBSD, OpenSolaris and Novell
Netware operating systems.
PV guests don't have any kind of
virtual emulated hardware, but
graphical console is still possible
using guest pvfb (paravirtual
framebuffer). PV guest graphical
console can be viewed using VNC
client, or Redhat's virt-viewer.
There's a separate VNC server in dom0
for each guest's PVFB.
Upstream kernel.org Linux kernels
since Linux 2.6.24 include Xen PV
guest (domU) support based on the
Linux pvops framework, so every
upstream Linux kernel can be
automatically used as Xen PV guest
kernel without any additional patches
or modifications.
See XenParavirtOps wiki page for more
information about Linux pvops Xen
support.
Xen Full virtualization (HVM)
Fully virtualized aka HVM (Hardware
Virtual Machine) guests require CPU
virtualization extensions from the
host CPU (Intel VT, AMD-V). Xen uses
modified version of Qemu to emulate
full PC hardware, including BIOS, IDE
disk controller, VGA graphic adapter,
USB controller, network adapter etc
for HVM guests. CPU virtualization
extensions are used to boost
performance of the emulation. Fully
virtualized guests don't require
special kernel, so for example Windows
operating systems can be used as Xen
HVM guest. Fully virtualized guests
are usually slower than
paravirtualized guests, because of the
required emulation.
To boost performance fully virtualized
HVM guests can use special paravirtual
device drivers to bypass the emulation
for disk and network IO. Xen Windows
HVM guests can use the opensource
GPLPV drivers. See
XenLinuxPVonHVMdrivers wiki page for
more information about Xen PV-on-HVM
drivers for Linux HVM guests.
KVM is not Xen at all, it is another technology, where KVM is a Linux native kernel module and not an additional kernel, like Xen. Which makes KVM a better design. the downside here is that KVM is newer than Xen, so it might be lacking some of the features.
Best Answer
Add this line to the beginning of ntpd.conf:
That will help it keep up. Here is a link to the vmware best practices for timekeeping. I know you're not using vmware, but much of it should still be applicable.