Cloudware Technologies - Guide to Cloud Computing for Business and Technology Managers - page 309

and open source. It has pioneered the paravirtualization concept, on
which the guest operating system, by means of a specialized kernel,
can interact with the hypervisor, thus significantly improving per-
formance. In addition to an open-source distribution, Xen currently
forms the base of commercial hypervisors of a number of vendors
including Citrix XenServer and Oracle VM.
c. KVM: The kernel-based virtual machine (KVM) is a Linux virtu-
alization subsystem. It has been part of the mainline Linux kernel
since version 2.6.20, thus being natively supported by several dis-
tributions. In addition, activities such as memory management and
scheduling are carried out by existing kernel features, thus making
KVM simpler and smaller than hypervisors, which take control of
the entire machine. KVM leverages hardware-assisted virtualiza-
tion, which improves performance and allows it to support unmodi-
fied guest operating systems; currently, it supports several versions
of Windows, Linux, and UNIX.
15.2 Types of Virtualization
15.2.1 Operating System Virtualization
The use of operating system virtualization or partitioning (such as IBM
LPARs) in cloud environments may help to solve security and confidenti-
ality problems, which would otherwise impair the acceptance of the cloud
approach. For this type of virtualization, which is also called container or
jails , the host operating system plays a major role. This is a concept where
multiple identical system environments or runtime environments, which are
completely isolated from each other, run under one operating system kernel.
Seen from the outside, virtual environments appear as autonomous systems.
All running applications use the same kernel, but they can only see the pro-
cesses belonging to the same virtual environment.
Mainly, Internet service providers (ISPs), who offer (virtual) root serv-
ers, prefer this kind of virtualization because it is associated with a minor
performance loss and a high degree of security. The drawback of operating
system virtualization is its reduced flexibility: while multiple independent
instances of the same operating system can be used simultaneously, it is
not possible to run different operating systems at the same time. Popular
examples of operating system virtualization are the container technology
from Sun Solaris, OpenVZ for Linux, Linux-VServer, FreeBSD Jails, and
Virtuozzo.