Xen is an open source hypervisor originally developed in the late 1990s as part of the XenoServers project. It was designed to enable efficient virtualization on non-virtualizable hardware through paravirtualization. While the core of Xen remains open source under the Xen.org project, many companies offer proprietary products based on Xen like Citrix's XenServer and Xen Cloud Platform. Xen uses a "thin hypervisor" model where the hypervisor has minimal components and relies on separate service domains to handle tasks like device drivers and management. This architecture provides strong isolation, flexibility in component selection, and good scaling capabilities. Xen finds uses in server virtualization, cloud computing, desktop virtualization, client virtualization, and secure application isolation.

1. Why Xen?
August 2010

2. A Brief History of Xen
• Originally part of the XenoServers project in the late '90s
o worldwide network of compute resources
• Keir Fraser worked on it as his PhD project
• 2002 - Xen Open Sourced
o First hypervisor to champion the idea of paravirtualization
o Full, efficient secure virtualization on non-virtualizable
hardware
• 2005 - XenSource formed to commercialize Xen
o Proprietary management tools for Open Source core
• 2006 - Microsoft and VMware adopt paravirtualization
• 2007 - Citrix acquires XenSource
o Separate Xen.org formed to manage open source project
• 2009 - Xen Cloud Platform
• 2010 - XenClient

3. Xen.org? Open Source? Citrix?
• Core of Xen is all Free Software
• Possible to put together a completely functional system
o many people do
• That gives you a commodity baseline
o coordinated by xen.org, contributors from everywhere
• Many companies offer products derived from the free base
o Citrix
o Oracle
o Novell
o etc
• They add support and advanced features

4. Xen Compared to...
• Xen has a "thin hypervisor" model
o hypervisor has no device drivers, very little smarts
o is just there to keep "domains" isolated
o ~2MB executable
o relies on service domains to do lots of its work
• VMware ESXi is similar
o but also has device drivers
o and base of management stack
o hardware support depends on VMware writing drivers
• Hyper-V very similar to Xen
o Designers explicitly modelled it after Xen
• KVM has a very different model
o Linux kernel as hypervisor

5. Xen Architecture

6. Hyper-V Architecture

7. ESXi Architecture

8. KVM Architecture

9. Let's look at that again...

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15. So what did all that mean?
• Xen keeps the hypervisor job separate from:
o management OS
o management stack
o device drivers
o guests
• All these components are interchangable
o mixed-and-matched
o choose the best OS to support your apps and hardware
• Strong isolation between these components
o especially with modern hardware support
o domains can be restarted as needed without taking out
system
• Good scaling
o Use your big machine without big domains

16. Different Use Cases
• Servers
o the old reliable: server consolidation
o standard open-source Xen
• Cloud
o efficient management of images
o rapid instancing
o Xen Cloud Platform (about 80% of XenServer)
• Hosted desktops
o much the same as servers, except
o need lots of magic goo to make it efficient
 Lots of Citrix products

17. More Use Cases
• Client virtualization
o Personal domain vs. Corporate locked-down domain
o needs to work on modern laptops/desktops
o needs most up-to-date driver support
o XenClient (free, mostly open source)
• Secure Applications
o launch individual apps and app clusters in their own
domains
o for example:
 random website browsing
 online banking
 shopping
 ...
o Invisible Things Labs' Qubes-OS: http://qubes-os.org/

18. More Info
● Web
● Home Page (http://www.xen.org)
● Documents
● Why Xen? - What is Xen? - How Does Xen Work?
● Support
● IRC ##xen and #xen-api (XCP Project)
● Xen Mailing Lists (http://lists.xensource.com)
Thanks to Jeremy Fitzhardinge for the creation of these excellent slides on Xen and
other virtualization alternatives