[Vm-dev] VM as a Xen guest

Colin Putney colin at wiresong.com
Mon Dec 3 18:41:40 UTC 2012

On Mon, Dec 3, 2012 at 10:15 AM, Javier Pimás <elpochodelagente at gmail.com>wrote:

> I never quite understood how Xen compares to things like VMWare or
> VirtualBox. Is it the same mechanism? What are the differences?

Basically, Xen is a hardware abstraction layer. It takes care of
communicating with devices and allocating resources to "domains". What it
doesn't provide is virtual hardware. To run on top of Xen you write to an
specific ABI, and make "hypercalls" which are analogous to applications
making syscalls into an OS kernel. Some operating systems, (e.g., Linux)
have been modified to be able to run in this mode, which is called

VMware and VirtualBox and KVM on linux provide what is called HVM, with
simulated hardware. The guest OS doesn't communicate with Xen via
hypercalls, it thinks it's running on actual hardware, dealing with
hardware interrupts etc.

Paravirtualization is more efficient than HVM, but it requires special
support from the guest OS.

So if SqueakNOS can run on real hardware, it should work fine in HVM. It
would probably need some work at the lower levels to support
paravirtualization and run directly on Xen.
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