To Grasp the Cloud—Fundamental Concepts - Deploying and Managing a Cloud Infrastructure - page 38

Minimal Footprint
The whole Xen Project can be packaged into a 1 MB project. This is because of the fact
that the Xen Project uses microkernel design with a limited memory footprint and limited
user interface (UI) elements.
PV Support
The Xen hypervisor is capable of firing up paravirtual VM instances. This adds to the effi-
ciency of VM instances initiated by Xen when compared with HVM-based instances. It also
enables Xen to initiate PV instances on hardware that does not support virtualization exten-
sions. Another option is to use fully virtualized hypervisor virtual machine (HVM) instances
that utilize PV class drivers, often called PVHVM device drivers. This eliminates the emula-
tion layer that pure HVM device drivers have to embed for networking, storage, and processor
hardware. This in turn delivers PV-like performance for HVM instances.
KVM (Open Source)
Kernel-based Virtual Machine, or KVM, is another popular open-source hypervisor
option. KVM is packaged with popular Linux distributions and can be configured to
run on top of the Linux distribution currently installed on the server. KVM has deep
integration and support for processors, networking, and storage hardware. It implements
a modular approach with the primitive set of modules shipped with Linux distributions
and an available additional set of modules that can be easily added on top of the vanilla
deployment. KVM distributes these modules as KVM kernel modules—for example, the
kvm-intel.ko or kvm-amd.ko modules for processor-specific support. KVM can be used
to initiate and manage multiple OS instances, including popular Linux distributions as
well as Microsoft Windows.
In 2008, Qumranet, the company behind KVM, was acquired by enterprise Linux
distributor Red Hat. Since then, KVM is used for Red Hat Enterprise Linux (RHEL)
enterprise virtualization server deployments.
OpenVZ (Open Source)
OpenVZ takes a different approach to virtualize the host server. It divides up the resources
into “containers,” much like what Docker is doing with application deployment virtualization.
The containers are fluid in the sense that a complete pool of memory space is available for all
containers, which means that a container may make use of the memory of another container
as long as it's available.
OpenVZ does not implement native driver support for disk, processor, and networking
devices but claims near native performance.
OpenVZ is popular among small and medium-level hosting providers who have deployed
it to for virtual private server (VPS) offerings.