Citrix XenServer and VMware vSphere are two leading server
virtualization platforms. This document provides a technical and
commercial comparison of XenServer and vSphere, including
installation and maintenance, OS support, high availability,
disaster recovery and other important virtualization concerns that
should be weighed when exploring virtualization solution options.
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FREE: VMware vSphere Hypervisor (Also known as ESXi)
ADV: Free VMware vSphere Client to administer the server
DISAD: Cannot be managed with vCenter or other management solutions
COMMERCIAL: VMware vSphere
ADV: advanced functionally including centralized management, live migration of virtual machines, automatic load balancing, business continuity, power management, and backup and restore capabilities for virtual machines.
Hence, The extra word “Hypervisor” in the product name is the mystery & decides which one is free & which one must be purchased.
Free can be upgraded by putting a license.