Re: [Xen-users] difference between Xen and Dom0 kernel

["Fajar A. Nugraha" <fajar@xxxxxxxxx>]

On Thu, Nov 12, 2009 at 3:47 PM, Tapas Mishra <tapas@xxxxxxxxxxxx> wrote:
> I want to know what is the difference between Xen and a Dom 0 kernel.
> Then using Xen hypervisor and xen tools what are they.

A hypervisor "is a piece of software/hardware platform-virtualization
software that allows multiple operating systems to run on a host
computer concurrently". These "mutiple operating systems" are called
domains (dom) in Xen terminology. In an over-simplified term, the
hypervisor will take care of allocating resources (CPU, memory, etc.)
between the multiple running OS. The xen hypervisor is usually
installed as /boot/xen*.gz on Linux dom0s. To control the xen
hypervisor you need xen userland tools (example: /usr/sbin/xm).

dom0 is special priviledged domain which has access to physical I/O
resources (storage, NIC) and makes it available for other domains
(domUs). It requires a special kernel (dom0 kernel) that supports Xen
and have the necessary drivers to access physical resources.

PV domUs are special kind of domUs which requires domU kernel to be
modified to support Xen, thus providing minimum performance penalty.
dom0 kernel can also be used as PV domU kernel.

HVM domU is a kind of domU that doesn't require kernel modification.
Most modern OS (e.g. Windows XP, Linux, Solaris 10) can run inside HVM
domU using the same installer that you use to run it on bare metal.
Usually it has higher performance penalty compared to PV domU.

Note that Linux KVM takes a different approach from Xen : the kernel
of priviledged domain also acts as hypervisor.
Also the future goal is to integrate Xen support into upstream so that
in the future you can have one kernel that can run on bare metal and
Xen dom0/domU.

-- 
Fajar

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