The Hyper-V Virtual Machine Heartbeat alarm is raised when the virtual machine has stopped responding to the hypervisor or is not responding at regular intervals.
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When an Information alarm is raised

The guest operating system is not installed or has not yet been contacted. The virtual machine is starting up, stopping, stopped, dormant or relocating.

When a Low severity alarm is raised

The heartbeat is degraded or the virtual machine is receiving maintenance.

When a Medium severity alarm is raised

The heartbeat is stressed or the virtual machine may have stopped responding.

When a High severity alarm is raised

The heartbeat has failed, is predicting failure or an error has been reported.