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== Full Virtualization vs. Para-Virtualization ==
Xen provides two approaches to virtualization - full virtualization and paravirtualization. Full virtualization provides complete abstraction between the hardware and the guest operating system. In this scenario, the guest operating system is provided a complete virtual physical environment in which to run and, as such, is unaware that it is running inside a virtual machine. One advantage of full virtualization is that the operating system does not need to be modified in order to run in a Xen virtualized environment. This means that proprietary operating systems such as Microsoft Windows can be run on Linux systems.<htmlet>xen</htmlet>
Disadvantages of full virtualization are that performance is slightly reduced as compared to paravirtualization and Xen requires CPUs with special virtualization support built in (such as Intel-VT and AMD-V) in order to perform full virtualization.
Paravirtualization requires that a guest operating system be modified to Xen hardware virtual machine (HVM) virtualization provides support for the virtualization. This typically means that of unmodified guest operating systems without the loss of performance inherent in the use of full virtualization (the different types of virtualization are limited to open source systems such as Linux. The advantage to described in the [[An Overview of Virtualization Techniques]] chapter of this approach is that a paravirtualized guest system comes closer to native performance than a fully virtualized guest, and book). In the latest case of host systems containing CPUs with virtualization CPU support is not needed, the HVM approach provides superior levels of performance when compared to full virtualization.
As mentioned previously, in In order to support full hardware virtualization, the CPU must include Intel-VT or AMD-V support. This can be verified using the following commands:
xm create *cfg -c<pre>Using config file "./hvmXenHVMGuest.cfg".Started domain xenhvmXenHVMGuest</pre>
Text replacement - "<htmlet>xen<htmlet>" to "<htmlet>xen</htmlet>"
<table border="0" cellspacing="0" width="100%"><tr><td width="20%">[[An Overview of Virtualization Techniques|Previous]]<td align="center">[[Xen hardware virtual machine (HVM) provides support for the virtualization Virtualization Essentials|Table of unmodified guest operating systems. Where ever possible it is better to run paravirtualized guests because Contents]]<td width="20%" align="right">[[Installing and Running Windows XP or Vista as a Xen HVM domainU guests run slightly slower than paravirtualized domainU guests Guest|Next]]</td><tr><td width="20%">An Overview of Virtualization Techniques<td align="center"><td width="20%" align="right">Installing and require that the host system contain a processor type with build in virtualization support. That said, if you need to virtualize an operating system which has not, Running Windows XP or cannot be modified to run Vista as a paravirtualized Xen guest (such as Microsoft Windows) then HVM virtualization is a good solution.domainU Guest</td></table><hr>
== Checking Hardware Support for Xen Hardware Virtual Machines (HVM) ==
For Intel CPUs:
</pre>
The ''disk = '' line needs to be modified to reflect the required disk and CD/DVDN DVD drive configuration. For example, if you have decided to use a physical disk accessible on the host system as /dev/sdb:
<pre>
== Booting the HVM Guest ==
The guest system can now be started using the ''xm create'' commandm command, for example:
<pre>
All being well the domainU guest will start with output to the Xen text console similar to the following:
and the installation process from the chosen designated media will begin.
== Connecting to the HVM dominU domainU Guest Graphical Console ==
If SDL was chosen for the graphical console then the console should appear when the guest starts up. If, on the other hand, VNC was selected and the HVM domainU was not configured to automatically start ''vncviewer'' it is now necessary to connect manually. By default the VNC port is 5900 + the ID of the domain to which you wish to connect (which can be obtained using the ''xm list'' command). For example, to connect to domain ID 10:
ps -ef | grep vnc
root 2992 2441 13 14:51 ? 00:00:00 /usr/lib/xen/bin/qemu-dm -d 11 -vcpus 1 -boot cd
-serial pty -acpi -domain-name xenhvm XenHVMGuest -net nic,vlan=1,macaddr=00:16:3e:2e:10:b0,model=rtl8139
-net tap,vlan=1,bridge=xenbr0 -vnc 127.0.0.1:0 -vncviewer
</pre>
As we can see from the above output, the Xen guest named ''xenhvmXenHVMGuest'' is accessible using 127.0.0.1:0. Using this information we would, therefore, connect as follows:
<pre>
boot="cd"
</pre>
<htmlet>xen</htmlet>
<hr>
<table border="0" cellspacing="0" width="100%">
<tr>
<td width="20%">[[An Overview of Virtualization Techniques|Previous]]<td align="center">[[Xen Virtualization Essentials|Table of Contents]]<td width="20%" align="right">[[Installing and Running Windows XP or Vista as a Xen HVM domainU Guest|Next]]</td>
<tr>
<td width="20%">An Overview of Virtualization Techniques<td align="center"><td width="20%" align="right">Installing and Running Windows XP or Vista as a Xen HVM domainU Guest</td>
</table>
