Is it possible to upgrade from 2.2 to 3.0?

There are two major platform changes in RHEV 3.0

1. RHEV Manager moved to RHEL
2. RHEV Hypervisor is now based on RHEL 6

This prevents in place upgrade, however a migration is possible in most cases.  

Red Hat will provide migration tool and assistance to customers that wish to migrate their existing environment to RHEV 3.

An example (simplistic) flow will look like:

1. Make sure your 2.2 is up to date with the latest Z-stream, both manager and guests drivers
2. Prepare a server with clean installation of RHEV Manager 3.0
3. Use the migration tool provided by Red Hat to move all the existing configuration to the new RHEV Manager
4. At this point you will have fully functional RHEV 3.0 system, with all the existing clusters working in compatibility mode to 2.2.
5. Next stage will be topgrade the hosts while moving them to a new cluster and then move the virtual machines to the new cluster.
   Note: There will be a grace period in which working in the compatibility mode clusters is supported (You can even add more resources like hosts and storage to these clusters) though you won't be able to enjoy the enhanced features and performance benefits of the RHEL 6 based hypervizors.  

Please stay tuned for more about this towards RHEV 3.0 GA