Shortly after the Christmas rush, someone asked me to explain the difference between Citrix XenServer, Microsoft Hyper-V, and VMware. After giving it a little thought, I came up with the following explanation. It was a bit unique, and thought that I would share it with you all.

To begin with, think of yourself around the holidays with a list of presents to buy at the Mall. If you have ten purchases from ten different stores to make this afternoon, it may take you a couple of hours to get through the entire list. However, if you had to bring five kids with you to the mall for the day, it would take you significantly longer to get through the same shopping list. The overhead of managing the childrens’ needs during the trip adds a good deal of overhead to your workload that impacts the amount of time you dedicate to present acquisition.

This scenario plays itself out when comparing Xen and Hyper-V to VMware’s ESX. VMware’s ESX is a ‘bare metal’ installation. This means that the hypervisor is installed directly onto the server. At only 32 MB in size, ESX is optimized to manage the Virtual Machine I/O and hardware translation with very little overhead. It also allows ESX to over allocate resources such as memory leveraging direct control over the physical resources.

With XenServer and Hyper-V, both installations use a base or core OS. This installed Operating System manages the hardware similar to what happens in any other server installation. The hypervisor is then installed and is run in the core OS. It is this hypervisor that manages the interaction between the hosted VMs and the hardware. The significant difference is that the core OS manages the hypervisor rather than having the hypervisor installed directly onto the hardware itself. With the OS management as a filter/management interface between the hypervisor and hardware, it offers a layer of complexity that is absent in the VMware ESX offering.

Similar to bringing along several kids with you to the mall, a hosted hypervisor will always have additional overhead associated with the core OS that you won’t have with VMware’s ESX. Additionally, because ESX controls the hardware directly, there is significant savings that can be realized in resource over allocation that can’t be found in hosted hypervisors. Hosted hypervisors can only allocate a subset of their physical resources to VMs, where ESX can optimize and prioritize the resources to get the most out of the physical assets.

So next time you are wondering about the difference between hypervisors, think of your last trip to the mall and smile.