XenServer have four different editions: Free, Advanced, Enterprise, Platinum. You can ask for free license at no cost but you have to renew it each year. The free edition lacks some features that are only available on the other three editions. We can easily access XenServer from XenCenter; we can even manage multiple XenServer from a single XenCenter.
Room for Improvement:
XenCenter is not available on Mac OS X or Linux, so we can only manage XenServer(s) from Windows. XenServer also lack of web based administration interface. We have to install XenServer on a dedicated machine, we can’t even have XenServer dual boot with another OS because XenServer installation will wipe the whole drive before installation. The processor that we use has to support virtualization (Intel-VT or AMD-V).
Good alternative of VMware ESXi, you can use free version to try the product first and perhaps move to the paid version if you need support or you need features that is not available in the free version.