I think this tip will help you to understand the difference between these status..
It means there is something wrong with the configuration of the hypervisor. The ovirt-engine can still communicate ok with the hypervisor though.
It can be a failure in the connectivity of hypervisor to any of the cluster ( ex: logical network, storage domain..etc ) components. So check hypervisor is capable of working with all the components defined for a cluster.
means that the Ovirt Engine cannot communicate with the hypervisor via vdsm.