However, looking at heart-rate (HR) itself will be misleading. Big swings in HR or high HR in general usually misinterpreted as having high HRV, when it's often the exact opposite. HRV is a measure of the variability of 1/(inter-beat time). So it's measuring the time-average of really small variations from a set point.
And counterintuitively, HRV is usually highest when the sympathetic (flight or fight) nervous tone and HR is lower. In those situations, the parasympathetic control dominates, and small wiggles caused by "noise on the wire" conducted directly to the heart's pacemaker via the vagus nerve increases HRV. Often times this is when HR is lower, and because higher HR or spikes in HR are often caused by increased sympathetic tone, this leads to a blunting of this "parasympathetic noise".
Source: https://journals.physiology.org/doi/full/10.1152/physiol.000...