Heartbeat needs some mechanism to fence or stonith a failed node. Two popular ways to do this are either to use IPMI or a network controllable power strip. Each node needs to have a mechanism available to reset any other node in the cluster. The example configuration in this document uses IPMI.
It is possible to configure PVFS and Heartbeat without a power control device. However, if you deploy this configuration for any purpose other than evaluation, then you run a very serious risk of data corruption. Without stonith, there is no way to guarantee that a failed node has completely shutdown and stopped accessing its storage device before failing over.