This Chapter provides a quantitative and qualitative comparison of four popular virtualization platforms, open-source hypervisors Xen, KVM and proprietary hypervisors VMware vSphere (ESXi), Microsoft Hyper-V. Cloud Computing is on Demand, Pay-per-use distributed computing service delivery model in which computing resources can be used as Utility like other utility such as water, electricity etc. as per requirement. Cloud computing has made it possible to provide virtually unlimited computing infrastructure i.e. IaaS on demand using virtualization technology. Intel and AMD have independently developed virtualization extensions to the x86 architecture referred to as hardware virtualization.