1. Running other code under LINUX
LINUX Virtualization
Running other code under LINUX

2. Environment Virtualization
Citrix/MetaFrame – Virtual desktop under Windows NT. aka Windows Remote Desktop Protocol
VNC, Dameware – virtual console.
XWindows – virtual console
Mac OS RDP – same as Windows.
WUBI.EXE

3. Hardware Emulation IBM – Virtual Machine (VM) OS Also using code:
Processor virtualization
Instruction set virtualization.
Slow. Usually requires OS (re-)installation
Examples: KVM (uses QEMU), Solaris Domains, DOSBox, DOSEmu, WINE

4. Processor Virtualization
“Native” or full virtualization: virtual machine that mediates between the guest operating systems and the native hardware. Certain protected instructions must be trapped and handled within the hypervisor because the underlying hardware isn't owned by an operating system but is instead shared by it through the hypervisor.
This form usually requires specially virtualization CPU processors (Intel, AMD) for performance. The only constraint is that the operating system must support the underlying hardware. Close to hardware-level performance.
The biggest advantage of full virtualization is that a guest OS can run unmodified. OS is usually “ported” to the hypervisor machine.
Examples: VMWare.

5. Processor Virtualization
Paravirtualization: uses a hypervisor for shared access to the underlying hardware but integrates virtualization-aware code into the operating system itself. Obviates the need for any recompilation or trapping because the operating systems themselves cooperate in the virtualization process.
Also requires specialized CPU hardware but not to the same level as “native” virtualization. Close to “native” performance.
Guest OS is (re-)installed to run virtualized.
Examples: LINUX Xen, User Mode LINUX (UML), Microsoft Virtual PC.

6. OS Level Virtualization
OS Level Virtualization: Virtualizes servers on top of the operating system itself - uses a single operating system and simply isolates the independent virtual servers from one another (code, memory, filesystems).
Doesn’t use special virtualization hardware
Uses a lot of underlying OS resources – memory, filesystem space.
Original concept – BSD “chroot jail”
Examples: LINUX: lxc “containers”. Solaris “Zones”

7. Hardware support for virtualization
IA-32 (Intel x86) supports four rings of protection, where level 0 (the highest privilege) typically runs the operating system, levels 1 and 2 support operating system services, and level 3 (the lowest level) supports applications.
In IA-32 (x86) architecture certain privileged-mode instructions do not trap, and can return different results based upon the mode. This is problematic when attempting to virtualize different operating systems at different levels.
Hardware vendors have recognized this shortcoming (and others), and have produced new designs that support and accelerate virtualization. Intel’s new virtualization technology supports hypervisors for both the x86 32-bit (VT-x) and Itanium® (VT-I) 64-bit architectures. The VT-x supports two new forms of operation, one for the VMM (root) and one for guest operating systems (non-root). The root form is fully privileged, while the non-root form is deprivileged (even for ring 0).
AMD is producing hardware-assisted virtualization technology, under the name Pacifica which Pacifica maintains a control block for guest operating systems that are saved on execution of special instructions. This allows a virtual machine (and its associated guest operating system) to run until the VMM regains control. Pacifica also amends address translation with host and guest memory management unit (MMU) tables.
These hardware virtualization can be used by a number of virtualization techniques, including Xen, VMware, User-mode Linux, and others.

8. Virtualization Products
Bochs and QEMU are PC emulators that allow operating systems such as Windows or Linux to be run in the user-space of a Linux operating system.
VMware is a popular commercial full-virtualization solution that can virtualize unmodified operating systems.
Xen is an open source paravirtualization solution that requires modifications to the guest operating systems but achieves near native performance by collaborating with the hypervisor.
Microsoft Virtual PC is a paravirtualization virtual machine approach.
User-mode Linux (UML) is another paravirtualization solution that is open source. Each guest operating system executes as a process of the host operating system.
coLinux, or Cooperative Linux, is a virtualization solution that allows two operating systems to cooperatively share the underlying hardware.
Linux-Contaimers (see also Docker)
The Linux KVM is virtualization technology that has been integrated into the mainline Linux kernel (not KVM kernel, but does require PAE support). Runs as a single kernel loadable module, a Linux kernel running on virtualization-capable hardware is able to act as a hypervisor and support unmodified Linux and Windows guest operating systems.