OpenVZ is container-based virtualization for Linux. OpenVZ creates multiple secure, isolated containers (otherwise known as VEs or VPSs) on a single physical server enabling better server utilization and ensuring that applications do not conflict. Each container performs and executes exactly like a stand-alone server; a container can be rebooted independently and have root access, users, IP addresses, memory, processes, files, applications, system libraries and configuration files.

The Xen® hypervisor, the powerful open source industry standard for virtualization, offers a powerful, efficient, and secure feature set for virtualization of x86, x86_64, IA64, PowerPC, and other CPU architectures. It supports a wide range of guest operating systems including Windows®, Linux®, Solaris®, and various versions of the BSD operating systems.

Enterprises looking to increase server utilization, consolidate server farms, reduce complexity, and decrease total cost of ownership are embracing server virtualization. The Xen® hypervisor is the fastest and most secure infrastructure virtualization solution available today, supporting a wide range of guest operating systems including Windows®, Linux®, Solaris®, and various versions of the BSD operating systems.

With Xen virtualization, a thin software layer known as the Xen hypervisor is inserted between the server's hardware and the operating system. This provides an abstraction layer that allows each physical server to run one or more "virtual servers", effectively decoupling the operating system and its applications from the underlying physical server.

The Xen hypervisor is a unique open source technology, developed collaboratively by the Xen community and engineers at over 20 of the most innovative data center solution vendors, including AMD, Cisco, Dell, HP, IBM, Intel, Mellanox, Network Appliance, Novell, Red Hat, SGI, Sun, Unisys, Veritas, Voltaire, and Citrix. Xen is licensed under the GNU General Public License (GPL2) and is available at no charge in both source and object format. Xen is, and always will be, open sourced, uniting the industry and the Xen ecosystem to speed the adoption of virtualization in the enterprise.

Paravirtualization fundamentally altered the way virtualization technology was architected. With this technology the virtual servers and hypervisor co-operate to achieve very high performance for I/O, CPU, and memory virtualization. The Xen hypervisor appears to the virtualized server as an idealized hardware abstraction layer that offers superb performance. In fact, the Xen hypervisor offers a smaller code base, greater security, and up to 10 times less overhead then alternative virtualization approaches
Comparison of Xen Paravirtualization implementation to other approaches to Paravirtualization

In addition, the Xen hypervisor uniquely takes advantage of hardware virtualization support from Intel® and AMD® processors to enable virtualized guests to run natively on the hardware while still achieving very high performance I/O. With alternative approaches, the hypervisor must binary patch running guests to prevent them from interacting with the hardware, resulting in high performance overhead, stability, and security risks. Moreover, this approach results in significant I/O performance impact.

Paravirtualization requires a tiny hypervisor code base (the Xen hypervisor is under 50 KLOC) that results in extremely low performance overhead, typically in the range of 0.1% to 3.5% for industry standard performance benchmarks. It also leverages all of the native Linux device drivers and therefore supports an extremely diverse set of drivers. Xen’s paravirtualized drivers run outside the core hypervisor, where they implement policy for resource sharing between multiple guests, providing fine-grained partitioning of I/O between multiple virtual servers. Another benefit of this approach is that drivers run at a lower protection level than Xen, making the hypervisor impervious to driver failure.

VMware software provides a completely virtualized set of hardware to the guest operating system. VMware software virtualizes the hardware for a video adapter, a network adapter, and hard disk adapters. The host provides pass-through drivers for guest USB, serial, and parallel devices. In this way, VMware virtual machines become highly portable between computers, because every host looks nearly identical to the guest. In practice, a system administrator can pause operations on a virtual machine guest, move or copy that guest to another physical computer, and there resume execution exactly at the point of suspension. Alternately, for enterprise servers, a feature called VMotion allows the migration of operational guest virtual machines between similar but separate hardware hosts sharing the same storage.

VMware Workstation, Server, and ESX take a more optimized path to running target operating systems on the host than emulators (such as Bochs) which simulate the function of each CPU instruction on the target machine one-by-one, or dynamic recompilation which compiles blocks of machine-instructions the first time they execute, and then uses the translated code directly when the code runs subsequently. (Microsoft Virtual PC for Mac OS X takes this approach.) VMware software does not emulate an instruction set for different hardware not physically present. This significantly boosts performance, but can cause problems when moving virtual machine guests between hardware hosts using different instruction-sets (such as found in 64-bit Intel and AMD CPUs), or between hardware hosts with a differing number of CPUs. Stopping the virtual-machine guest before moving it to a different CPU type generally causes no issues.

VMware's products use the CPU to run code directly whenever possible (as, for example, when running user-mode and virtual 8086 mode code on x86). When direct execution cannot operate, such as with kernel-level and real-mode code, VMware products re-write the code dynamically, a process VMware calls "binary translation" or BT. The translated code gets stored in spare memory, typically at the end of the address space, which segmentation mechanisms can protect and make invisible. For these reasons, VMware operates dramatically faster than emulators, running at more than 80% of the speed that the virtual guest operating-system would run directly on the same hardware. VMware claims an overhead as small as 3% to 6% for computationally-intensive applications.

VMware's approach avoids some of the difficulties of virtualization on x86-based platforms. Virtual machines may deal with offending instructions by replacing them, or by simply running kernel-code in user-mode. Replacing instructions runs the risk that the code may fail to find the expected content if it reads itself; one cannot protect code against reading while allowing normal execution, and replacing in-place becomes complicated. Running the code unmodified in user-mode will also fail, as most instructions which just read the machine-state do not cause an exception and will betray the real state of the program, and certain instructions silently change behavior in user-mode. One must always rewrite; performing a simulation of the current program counter in the original location when necessary and (notably) remapping hardware code breakpoints.

Although VMware virtual machines run in user-mode, VMware Workstation itself requires the installation of various drivers in the host operating-system, notably to dynamically switch the GDT and the IDT tables.

The VMware product line can also run different operating systems on a dual-boot system simultaneously by booting one partition natively while using the other as a guest within VMware Workstation.