Understanding Full Virtualization, Paravirtualization, and Hardware Assist

Short Description
In 1998, VMware figured out how to virtualize the x86 platform, once thought to be impossible, and created the market for x86 virtualization. The solution was a combination of binary translation and direct execution on the processor that allowed multiple guest OSes to run in full isolation on the same computer with readily affordable virtualization overhead. The savings that tens of thousands of companies have generated from the deployment of this technology is further driving the rapid adoption of virtualized computing from the desktop to the data center. As new vendors enter the space and attempt to differentiate their products, many are creating confusion with their marketing claims and terminology. For example, while hardware assist is a valuable technique that will mature and expand the envelope of workloads that can be virtualized, paravirtualization is not an entirely new technology that offers an “order of magnitude” greater performance.

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Content
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Overview of x86 Virtualization
The term virtualization broadly describes the separation of a service request from the underlying physical delivery of that service. With x86 computer virtualization, a virtualization layer is added between the hardware and operating system as seen in Figure 2. This virtualization layer allows multiple operating system instances to run concurrently within virtual machines on a single computer, dynamically partitioning and sharing the available physical resources such as CPU, storage, memory and I/O devices.
As desktop and server processing capacity has consistently increased year after year, virtualization has proved to be a powerful technology to simplify software development and testing, to enable server consolidation, and to enhance data center agility and business continuity. As it turns out, fully abstracting the operating system and applications from the hardware and encapsulating them into portable virtual machines has enabled virtual infrastructure features simply not possible with hardware alone. For example, servers can now run in extremely fault tolerant configurations on virtual infrastructure 24×7x365 with no downtime needed for backups or hardware maintenance. VMware has customers with production servers that have been running without downtime for over three years.
For industry standard x86 systems, virtualization approaches use either a hosted or a hypervisor architecture. A hosted architecture installs and runs the virtualization layer as an application on top of an operating system and supports the broadest range of hardware configurations. In contrast, a hypervisor (bare-metal) architecture installs the virtualization layer directly on a clean x86-based system. Since it has direct access to the hardware resources rather than going through an operating system, a hypervisor is more efficient than a hosted architecture and delivers greater scalability, robustness and performance. VMware Player, ACE, Workstation and Server employ a hosted architecture for flexibility, while ESX Server employs a hypervisor architecture on certified hardware for data center class performance.
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