XenServer 6.1 introduced formal support for IPv6 in XenServer guest VMs (maintained with 6.5). Customers already used it with e.g. 6.0 but the 6.1 release notes list this as new official feature: IPv6 Guest Support: enables the use of IPv6 addresses within guests allowing network administrators to plan for network growth.

Full support for IPv6 (i.e. assigning the host itself an IPv6 address) will be addressed in the future.