This section describes Xen, XenAPI, XenServer, and XCP, their differences, and how to use them with OpenStack. After you understand how the Xen and KVM architectures differ, you can determine when to use each architecture in your OpenStack cloud.

Xen. A hypervisor that provides the fundamental isolation between virtual machines. Xen is open source (GPLv2) and is managed by Xen.org, an cross-industry organization. Xen is a component of many different products and projects. The hypervisor itself is very similar across all these projects, but the way that it is managed can be different, which can cause confusion if you're not clear which tool stack you are using. Make sure you know what tool stack you want before you get started. Xen Cloud Platform (XCP). An open source (GPLv2) tool stack for Xen. It is designed specifically as a platform for enterprise and cloud computing, and is well integrated with OpenStack. XCP is available both as a binary distribution, installed from an iso, and from Linux distributions, such as xcp-xapi in Ubuntu. The current versions of XCP available in Linux distributions do not yet include all the features available in the binary distribution of XCP. Citrix XenServer. A commercial product. It is based on XCP, and exposes the same tool stack and management API. As an analogy, think of XenServer being based on XCP in the way that Red Hat Enterprise Linux is based on Fedora. XenServer has a free version (which is very similar to XCP) and paid-for versions with additional features enabled. Citrix provides support for XenServer, but as of July 2012, they do not provide any support for XCP. For a comparison between these products see the XCP Feature Matrix. Both XenServer and XCP include Xen, Linux, and the primary control daemon known as xapi. The API shared between XCP and XenServer is called XenAPI. OpenStack usually refers to XenAPI, to indicate that the integration works equally well on XCP and XenServer. Sometimes, a careless person will refer to XenServer specifically, but you can be reasonably confident that anything that works on XenServer will also work on the latest version of XCP. Read the XenAPI Object Model Overview for definitions of XenAPI specific terms such as SR, VDI, VIF and PIF.

A Xen host runs a number of virtual machines, VMs, or domains (the terms are synonymous on Xen). One of these is in charge of running the rest of the system, and is known as "domain 0," or "dom0." It is the first domain to boot after Xen, and owns the storage and networking hardware, the device drivers, and the primary control software. Any other VM is unprivileged, and are known as a "domU" or "guest". All customer VMs are unprivileged of course, but you should note that on Xen the OpenStack control software (nova-compute) also runs in a domU. This gives a level of security isolation between the privileged system software and the OpenStack software (much of which is customer-facing). This architecture is described in more detail later. There is an ongoing project to split domain 0 into multiple privileged domains known as driver domains and stub domains. This would give even better separation between critical components. This technology is what powers Citrix XenClient RT, and is likely to be added into XCP in the next few years. However, the current architecture just has three levels of separation: dom0, the OpenStack domU, and the completely unprivileged customer VMs.

A Xen virtual machine can be paravirtualized (PV) or hardware virtualized (HVM). This refers to the interaction between Xen, domain 0, and the guest VM's kernel. PV guests are aware of the fact that they are virtualized and will co-operate with Xen and domain 0; this gives them better performance characteristics. HVM guests are not aware of their environment, and the hardware has to pretend that they are running on an unvirtualized machine. HVM guests have the advantage that there is no need to modify the guest operating system, which is essential when running Windows. In OpenStack, customer VMs may run in either PV or HVM mode. However, the OpenStack domU (that's the one running nova-compute) must be running in PV mode.

When you deploy OpenStack on XCP or XenServer, you get something similar to this: Key things to note: The hypervisor: Xen Domain 0: runs xapi and some small pieces from OpenStack (some xapi plugins and network isolation rules). The majority of this is provided by XenServer or XCP (or yourself using Kronos). OpenStack VM: The nova-compute code runs in a paravirtualized virtual machine, running on the host under management. Each host runs a local instance of nova-compute. It will often also be running nova-network (depending on your network mode). In this case, nova-network is managing the addresses given to the tenant VMs through DHCP. Nova uses the XenAPI Python library to talk to xapi, and it uses the Management Network to reach from the domU to dom0 without leaving the host. Some notes on the networking: The above diagram assumes FlatDHCP networking (the DevStack default). There are three main OpenStack Networks: Management network - RabbitMQ, MySQL, etc. Please note that the VM images are downloaded by the XenAPI plugins, so please make sure that the images can be downloaded through the management network. It usually means binding those services to the management interface. Tenant network - controlled by nova-network. The parameters of this network depend on the networking model selected (Flat, Flat DHCP, VLAN). Public network - floating IPs, public API endpoints. The networks shown here need to be connected to the corresponding physical networks within the data center. In the simplest case, three individual physical network cards could be used. It is also possible to use VLANs to separate these networks. Please note, that the selected configuration must be in line with the networking model selected for the cloud. (In case of VLAN networking, the physical channels have to be able to forward the tagged traffic.)

The host-aggregates feature enables you to create pools of XenServer hosts to enable live migration when using shared storage. However, you cannot configure shared storage.

Here are some of the resources available to learn more about Xen: Citrix XenServer official documentation: http://docs.vmd.citrix.com/XenServer. What is Xen? by Xen.org: http://xen.org/files/Marketing/WhatisXen.pdf. Xen Hypervisor project: http://xen.org/products/xenhyp.html. XCP project: http://xen.org/products/cloudxen.html. Further XenServer and OpenStack information: http://wiki.openstack.org/XenServer.