oVirt is a community project, sponsored by Red Hat. It provides installers, a web GUI, and associated utilities and libraries for the management of virtual machines (KVM), storage (Gluster) and Networking.
RHEV is a product sold by Red Hat, run on top of Red Hat Enterprise Linux 6 (as of this writing) and which comes designated as "enterprise-ready". It is officially supported by Red Hat.
Fedora is rapidly developed by both Red Hat employees and community contributors and the technologies make their way into future Red Hat Enterprise Linux releases. oVirt has the same relationship to RHEV. Rapid development takes place in the oVirt upstream and over time, the improvements will be released as a new version of RHEV and supported by Red Hat.
If your interest is geared at running virtual machines in a production or enterprise environment, backed with the support of Red Hat, RHEV is where you'll look. If you require more cutting edge features, or are interested in contributing to future releases, oVirt is what you want. Red Hat does not, and will not, offer support on oVirt.