Finding the appropriate virtualization technology to fit an organization's requirements depends on the needs of the environment, combined with the feature sets of each virtualization architecture. Oftentimes, organizations find that a single approach doesn't complete the picture and multiple virtualization technologies are required.

The two basic server virtualization architectures, hardware and operating systems (OS), are each designed with different workloads and environments in mind. Some environments are more suited to OS virtualization, specifically those with naturally high levels of homogeneity or those with requirements for high-performance or consolidation ratios. Other environments, where varying OS support is a high priority, may work better with hardware virtualization.

• Hardware Virtualization: This approach virtualizes the entire server from the hardware up to the software stack. This architecture solves many problems associated with consolidation of different computing environments on a single physical server.
• OS Virtualization: OS virtualization resides on top of an OS and leverages the underlying hardware and software. This concept provides ultra-high consolidation ratios and resource efficiency for many computing environments.

It's All About the Layer
When it comes to virtualization products and solutions, it's all about the layer. The "layer" refers to the level at which the virtualization occurs. Hypervisor-based hardware virtualization solutions include their layer of processing either above an existing operating system or directly on top of the hardware, an approach known as "bare metal."

Putting the hypervisor in close proximity to the physical hardware enables it to robustly support multiple operating systems on a single physical host. Those individual virtual machines are fully independent from each other and don't rely on any other machines for their composition or processing.

This complete isolation from individual virtual machines is hardware virtualization's greatest strength and greatest area of management complexity. As virtual machines are effectively segregated units, they must be managed similarly to segregated physical machines. Patching, software management, resource allocation, and security are all horizontally scaled as the number of virtual machines increases. This can lead to a virtual "sprawl" of devices to manage as the environment scales. In effect, the same level of management effort is required as compared with physical machines because each virtual machine is managed individually.

Compare this scenario to what is experienced in environments that leverage OS virtualization, or containers, where the layer operates atop an existing full operating system, such as Microsoft Windows or Red Hat Linux. Rather than making use of a hypervisor layer, it uses the virtualization layer as a mechanism for extending the resources of a single operating system to multiple, isolated, additional virtual systems that all reside on the same host.