Selecting VMware ESX or VMware ESXi
One of the first major decisions that you need to make when planning VMware vSphere 4 is whether to use VMware ESX or VMware ESXi. If you choose ESXi, you must also choose between ESXi Installable and ESXi Embedded. To make this decision, though, you must first understand some of the architectural differences between ESX and ESXi.
VMkernel's Double Personality
The hypervisor is the software that runs on the bare metal and provides the virtualization functionality. Although VMkernel is commonly used as the name for VMware's bare-metal hypervisor found in both ESX and ESXi, two distinct components are actually involved. VMkernel manages physical resources, process creation, I/O stacks, and device drivers. The virtual machine monitor (YMM) is responsible for actually executing commands on the CPUs, performing binary translation (BT) or programming VT/SVM hardware, and so on, and is instanced-meaning that a separate VMM exists for each virtual machine. Although these two components are indeed separate and distinct, for the sake of simplicity I'll continue to refer to both of them as VMkernel unless there is a clear need to distinguish them.
ESX and ESXi share the same 64-bit, bare-metal hypervisor at their cores (commonly known as VMkernel). Both ESX and ESXi can be managed by vCenter Server, and both ESX and ESXi support advanced virtualization functionality such as VMotion, Storage VMotion, VMware Distributed Resource Scheduler (DRS), VMware High Availability (HA), and VMware Fault Tolerance (FT).
VMware ESX incorporates a customized 64-bit management interface, known as the Service Console, which provides an interface for administrators to use to interact with the hypervisor. This built-in service console, based on Linux, provides a place for third-party applications or agents to execute and allows vSphere administrators to run command-line configuration tools and custom scripts.
VMware ESXi omits the Service Console. Instead, ESXi is a hypervisor-only deployment that requires just 32MB of space. By omitting the Service Console, ESXi also eliminates the potential security vulnerabilities that are contained within that customized Linux environment, as well as dramatically shrinks its footprint. This minimized footprint is what enables ESXi to be distributed in two different versions: ESXi Installable, which can be installed onto a server's hard drives; and ESXi Embedded, which is intended to run from a Universal Serial Bus (USB)-based flash device. Aside from their intended deployment model, ESXi Installable and ESXi Embedded are the same; they share the same underlying architecture and code. I'll just refer to both of them as ESXi except where it's necessary to distinguish between ESXi Installable and ESXi Embedded. Although IJSX enjoys much broader support from third-party tools like backup or monitoring tools, VMware has made no secret that the future of its bare-metal hypervisor lies with ESXi. Therefore, when evaluating ESX vs. ESXi for your deployment, be sure to consider ESXi support if you plan to use third-party tools in your deployment.