Citrix XenServer Free Edition is available to use at no cost and contains all the features needed to create a virtual infrastructure. The XenServer Free Edition is available for free production use with no restrictions or time limits; all you need to do is activate the product within 30 days of first installing it to register your intent to use it with Citrix. Activation is a simple process that takes only a few minutes. For information about activating XenServer Free Edition, see Activate a Free XenServer Product.
Three paid-for XenServer Editions — Advanced, Enterprise and Platinum — provide a set of management and automation capabilities that extend the capabilities of the free Citrix XenServer platform. For information about the features available in different XenServer editions, click here.
All paid editions of the product use Citrix centralized licensing, and you must allocate licenses using a Citrix License Server, just as with other Citrix components. All XenServer Editions licenses must be installed on to a separate Citrix Licensing Server, and maintained and controlled using the Citrix License Management Console. In this way, XenServer shares the same licensing model as other Citrix products, and the licensing server can be shared across other Citrix products. For details of how to install and run Citrix Licensing, look at the online documentation on the Citrix eDocs website.
For information about managing licenses, see Managing Licenses in XenServer 6.1.0 and earlier.
Citrix V6 licensing has built-in timeout technology. If a XenServer host cannot contact the license server, for example, due to problems with the license server hardware or software or network failures, there is a 30 day grace period that allows XenServer hosts to continue operations as if they were still in communication with the license server. After a startup license is checked out by a XenServer host, the XenServer and the license server exchange "heartbeat" messages every five minutes to indicate to each other that they are still up and running. If XenServer and the license server fail to send or receive heartbeats, XenServer lapses into the licensing grace period and licenses itself through cached information. The grace period is 30 days and when the grace period runs out, XenServer stops accepting connections. After communication is re-established between XenServer and the license server, the grace period is reset. The grace period takes place only if the XenServer has successfully communicated with the license server at least once.