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Picking a hypervisor technology is one of the most important decisions an administrator can make. When making such a decision, keep in mind that whichever hypervisor you choose is going to be around for a while, since hypervisor replacement is a difficult process, one that is often delayed until forklifts arrive for the systems.
Anyone familiar with virtualized systems already knows that each hypervisor technology has a different level of complexity and functionality. This can complicate things, but thoroughly research different hypervisor vendors can narrow your options considerably. Let's look at the candidates.
VMware ESX/ESXi is the leader in bare-metal hypervisors today, dwarfing the combined share of all other hypervisor vendors. It is surrounded by an extensive ecosystem of VMware and third-party tools, with vSphere and NSX network virtualization being the most important. These are well-tested and mature products, which contributes to their market share, though VMware's vSphere is priced higher than competing products.
VMware continues to add functionality to its products, but its products have also become more complex over time. This level of complexity requires specialized skills to manage, so if you choose to use ESX/ESXi, you may spend more on staffing and training. Add this to the high license fees for all products, and it becomes apparent why the industry generally regards VMware as a bit overpriced. This is troubling for VMware, since hypervisor virtualization is under attack from containers and also the cloud, an area where VMware has struggled to gain traction.
VMware has a very large and mature installed base, which gives it the advantage of customer loyalty -- which could also be characterized as lock-in. Still, the company recognizes threats to its standing and now supports Docker and OpenStack, among others.
ESX/ESXi offers excellent functionality with a rich ecosystem, security with NSX and overall stability. The only downside is that the total cost of ownership (TCO) is high due to licensing and training costs.
KVM is an open source bare-metal hypervisor for Linux with a broad list of supported guest OSes, including popular Linux distributions, Solaris and Windows Server. Red Hat delivers a supported version of KVM and has taken on the challenge of improving ease-of-use by simplifying operations and adding deployment tools.
Red Hat has taken the automation and configuration tools from Ansible and addressed scalability and portability, using Ansible's modular approach to mash up a variety of offerings, from virtual clusters to clouds.
KVM is attracting business from both VMware and Microsoft Hyper-V customers, while customized KVM flavors provided by Scale Computing and Nutanix are finding sizable niches.
In terms of functionality, KVM is very good; it leans toward automation rather than heavily manual tools. It also performs well in terms of stability, as the user community is very active and issues are addressed quickly. KVM's TCO is low and effective -- KVM comes standard in Linux version 2.6.20 and later.
Available as either a bare-metal stand-alone product or as an installable for Windows Server, Hyper-V is not as advanced as VMware, though the ubiquity of Windows Server assures that it is one of the top hypervisors. Microsoft shops planning virtualization naturally gravitate toward Hyper-V. The downside to this is that after rapid adoption, the installed base has effectively peaked.
Microsoft's heavy investment in cloud has affected the Hyper-V environment. Azure Stack is on its way to release and raises the question of how far to take virtualization versus entering fully into the cloud. It seems likely that the Microsoft base will lean toward Azure Stack, with its full integration with the public Azure cloud, which might mean that Hyper-V itself loses ground going forward.
Although Hyper-V offers excellent stability and has a middle range TCO, VMware is still the more popular option.
Citrix was an early virtualization vendor and has a sizeable following, however, the Citrix XenServer hypervisor appears to lag behind alternatives when it comes to features and suffers from some performance issues. XenServer adds significant overhead compared to its competitors.
Like Red Hat, Citrix has turned what was originally an open source project into a family of commercial offerings. Due to its early start, XenServer is a little more mature than KVM, but is evolving more slowly. Xen still has some advantage in near-native drivers, especially for Windows, but KVM has an edge with Linux.
Oracle offers a hypervisor technology based on the open source Xen, but this lacks many of the features offered by other leading hypervisor vendors.
In short, the Citrix XenServer hypervisor is a stable, mature product with decent functionality that comes at a low cost, but its ecosystem cannot keep up with other leading hypervisors.
Which comes out on top?
At the end of the day, VMware is probably the best option for heavy virtualization users, and Hyper-V for those in Microsoft shops. For those looking to save money, KVM is a good choice for Linux-based platforms, and Xen for platforms that mix Linux and Windows. Of course, there are exceptions to each of these use cases and, if one hypervisor technology doesn't work for you, you can migrate to another -- though this often comes at a cost.
The most interesting aspect to this debate is the looming specter of the cloud. The cloud has created a disturbance in the world of virtualization, with its automation and agility benefits threatening to overshadow the ecosystem advantages of on-premises hypervisors. Containers also pose a threat to hypervisors. For example, Intel Clear Containers provide thin hypervisors and could eliminate the need for ESXi or KVM. A future with cloud orchestration and management tools deploying containers seems possible, even though containers are still at the border from sandbox to mainstream use.
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