A virtual machine (VM) is a software container (sometimes called a “guest”) that runs on a host physical computer and that behaves as if it were a physical computer itself. VMs consist of an operating system plus CPU, memory (RAM) and networking resources, and software applications.

A template is a virtual machine encapsulated into a file, making it possible to rapidly deploy new VMs. Each template contains installation metadata—the setup information needed to create a new VM with a specific guest operating system, and with the optimum storage, CPU, memory and virtual network configuration.

You can create new VMs in XenCenter in a number of different ways:

• The New VM wizard takes you step by step through the process of creating a new VM from a template or a snapshot, allowing you to configure operating system, CPU, storage, networking and other parameters.
• You can bypass the New VM wizard and create an “instant VM” based on a custom VM template that specifies all of the required VM configuration parameters. You simply select your preconfigured template in XenCenter then right-click and click Instant VM from template . This mode of unattended VM installation can be useful for deploying large numbers of identical VMs.
• You can copy (or “clone”) an existing VM.
• You can import a VM that has been previously exported.

XenServer PV drivers – XenServer Tools

VMs in a XenServer environment may be fully virtualized (HVM) or paravirtualized:

• In HVM (hardware-assisted virtualization or Hardware Virtual Machine) mode, the VM is fully virtualized and can run at near-native processor speeds on virtualization-enabled hardware, without any modification to the guest operating system. Windows guests are always HVM (there’s no “Xen PV kernel” for Windows) but as long as you install Tools, you’re getting the best performance.
• In paravirtualized (non-HVM) mode, the guest operating system is tuned and optimized to run in a virtual environment, independent of the underlying processor capabilities. The result is better performance and greater flexibility.

Paravirtualized (PV) drivers are available for Windows and Linux VMs to enhance disk and network performance. These drivers are supplied in the XenServer Tools package and should be installed on all new VMs – see Installing XenServer Tools. XenServer features such as VM migration and historical performance data tracking are only available on VMs that have XenServer Tools installed.

Using templates

There are 3 types of templates:

Basic templates:

• they are just like an empty vessel, with just virtual hardware configuration.
• No Operating System

Full Templates:

• Full copies of OS
• Recommended Settings
• New packages can be added using apt-get utility after installing OS

Custom templates:

• Created from any VM
• can be duplicated
• Basic changes allowed

A number of different templates are supplied with XenServer, and these contain all the various configuration settings needed to install a specific guest operating system on a new VM. You can also create your own customized templates configured with the appropriate guest operating system, memory, CPU, storage and network settings, and use them to create new VMs. See the XenServer Virtual Machine User’s Guide for a list of the templates/operating systems supported at this release, and for detailed information about the different install mechanisms on Windows and Linux.

You can view the XenServer templates supplied with the product and any custom templates that you create in the Resources pane.

In Server View, you can control whether or not XenServer and Custom templates are shown in the Resources pane:

• To show standard XenServer VM templates: on the View menu, click Server View and then click to select XenServer Templates; to hide templates, click to remove the check mark.
• To show custom VM templates: on the View menu, click Server View and then click to select Custom Templates; to hide custom templates, click to remove the check mark.

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The process for creating and deploying a new VM from a custom template is as follows:

1.Build a new VM from a basic template 2.Customize the VM, ie, install services, load updates, etc. 3.Run SYSPrep to prepare the VM for cloning as XenServer does not provide a “Customization Specifications” found in VMware ESX 4.Once shutdown, convert the VM into a template 5.Launch the New VM Wizard, and select the custom template as the source 6.Boot the VM, run through the Windows Mini-Setup wizard, and then run any post image process to prepare the VM for production.