Can HRV be calculated from simple heart rate?

In short, no.

Heart rate (HR) is measured in beats per minute. It does not require exact times – just the average of the beats in a given time period. For example, 60 beats per minute HR could mean 1 beat per second or it could mean an average of 1 beat every 0.5s, 1.5s, 0.5s, 1.5s, etc.

While heart rate focuses on the average beats per minute, heart rate variability (HRV) measures the specific changes in time (or variability) between successive heartbeats. The time between beats is measured in milliseconds (ms) and is called an “R-R interval” or “inter-beat interval (IBI).”

The recommended minimum number of times per second that a heart rate variability sensor should sample for a beat is 250. This can be referred to as 250Hz. Any monitor with a lower sample rate should not be considered HRV-accurate.