Xen Hypervisor

Xen is a bare metal hypervisor, it starts immediately after BIOS is launched. It operates directly on the host's hardware and doesn't need any OS to be installed on, thus making it possible for host hardware to install numerous guest operating systems and to run them simultaneously. The guests use Xen interface for almost all their hardware requests, for example, I/O, CPU and disk. Xen provides open source code and can be utilized as a core virtualization engine. This hypervisor operates directly on x86-64, x86, Power PC, Itanium, IA-64, SPARC, and ARM processors.

Xen hypervisor launches Domain0 Guest, also called dom0, when the system initially boots. Hardware is directly available to Domain0 Guest and it has access to device drivers for other domains. It can also launch new domains. Domain Guests named domU are ran and managed by dom0 and work independently on the system. Such guests are either launched and managed by the means of a distinct modified OS known as paravirtualizion, or with the help of unmodified OS working on specially developed virtualization hardware (AMD-V and Intel VT) known as hardware virtual machine (HVM).

Citrix XenServer benefits:

There is complete isolation between all the system elements.
When used with high-end hardware and domains, Xen restarts alone allowing the whole system to continue its operations and processes;
XEN hypervisor allows some modified OS to run as the dom0;
Scalability
Security

Security in Xen is ensured by the wide range of features:

Guest isolation: each domU guest is separated from other domU guests, so they cannot access memory and networking connections of each other.
Privileged access: dom0 is given the privileged access to the hardware and device drivers.
OS separation: since the hypervisor is isolated from OS, one cannot utilize it for attacking the operating system.
Small code base.

Learn more about Citrix XenServer:

StarWind iSCSI SAN Solution for Citrix XenServer