The free software includes the advanced features, like migration of virtual machines from one host to another without interrupting the services, and active directory integration.
Xen hypervisor is open source.
Xen hypervisor is installed on bare metal and provides high performance.
Real time performance monitoring
Hot swappable disks and NICs
Unlimited Virtual machines, Memory
Virtual Machine template
Room for Improvement:
Xenservers need a 64 bit system with IntelVT or AMD V technology for windows guests.
Management console only supports Windows
Windows guests are well supported, but Linux guests have limited support.
Its list of supported Linux guests is quite small.
Xenserver provides numerous features like, Xenmotion - live migration of virtual machines, real time performance monitoring, high performance using bare metal installation, and it's all absolutely free. It provides unlimited virtualization . Xenserver has a unique workload balancing technique which captures CPU, memory, network, and disk data. It may lack certain features, but it can satisfy many environments. In my opinion, Xenserver provides better features for free, making it the best for home, lab, and research purposes.