Introduction to Virtualization - Oracle Solaris 10 System Virtualization Essentials - page 34

Oracle VM Server for x86, xVM Hypervisor and Citrix Xenserver are com-
mercial implementations of the open-source Xen hypervisor for x86 systems. Xen
supports a variety of guest operating systems, but differs in architecture from
VMware ESX: Xen uses specialized guest domains for parts of the virtualization
infrastructure. A specially privileged “dom0” guest, running OpenSolaris, Linux,
or BSD, provides a management control point and typically provides virtual I/O
to other guests.
The Oracle VM Server for SPARC (previously called Sun Logical Domains) fea-
ture, which is discussed in detail later, is a SPARC hypervisor on chip multithread-
ing (CMT) servers used to support guest systems running Solaris. The VMC is
called the control domain . Virtual devices are implemented by the control domain
or one or more specialized VEs called service domains and I/O domains . Service
domains or I/O domains can be grouped into HA pairs to improve availability of
I/O devices. System administrators can also assign devices directly to VEs.
IBM's PowerVM Hypervisor is a combination of firmware and software that
creates and manages LPARs (logical partitions) on Power5 and Power6 CPU cores.
These systems also support a virtualization technology called Micro-Partitioning
that can run multiple OS instances on a CPU core.
Partitions may run AIX or Linux operating systems. PowerVM offers VIO parti-
tions and direct device assignment.
1.2.2.3 Type 2 Hypervisors
Type 2 hypervisors run within a conventional operating system environment, en-
abling virtualization within an OS. The computer's OS (e.g., Oracle Solaris, Linux
distributions, Microsoft Windows) boots first and manages access to physical re-
sources such as CPUs, memory, and I/O devices. The Type 2 hypervisor operates
as an application on that OS. Like the Type 1 hypervisor, the Type 2 hypervisor
may make use of or require hardware-assist technology to decrease overhead at-
tributable to the hypervisor.
Type 2 hypervisors do have some disadvantages. For example, they must de-
pend on the services of a hosting operating system that has not been specifically
designed for hosting virtual machines. Also, the larger memory footprint and CPU
consumed by unrelated features of a conventional OS may reduce the amount of
physical resources remaining for guests.
The primary advantage of Type 2 hypervisors for desktop systems is that the
user can continue to run some applications, such as e-mail, word processing, and
software development software, using a favorite OS and its tools, running without
performance penalty. Other advantages include the ability to leverage features
provided by the OS: process abstractions, file systems, device drivers, web servers,
debuggers, error recovery, command-line interfaces, and a network stack. Similar