Debian – Open Xen Manager and XenCenter server connection

It's not entirely clear to me how you got PPTP working to the Windows XP server when you have forwarded all port 1723 traffic to the Debian Squeeze server. You likely can only "VPN" to the WinXP server from within your local LAN, which seems to be of limited usefulness.

Regardless, PPTP requires not only TCP port 1723 traffic, but also GRE protocol. Is your router capable of handling GRE tunnels correctly? If it's an ordinary consumer-grade router, then I suspect not. And even if it is, GRE is esoteric enough that finding help may be difficult.

In your case I recommend trying a VPN solution that uses only TCP and/or UDP transports, since those protocols are ubiquitous and well-known. OpenVPN is one such VPN solution, and it is available for all the major OSes (Win, Mac, Linux, *BSD).

Depending on what you're trying to accomplish, another possibility is to run sshd on the Debian server, e.g.:

apt-get install openssh-server

All the major OSes have free ssh clients capable of creating tunnels over an ssh connection.

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.

This is from http://wiki.xenproject.org/wiki/XenOverview

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.