You will have numerous sensor available to you and they break two types. One type uses pulse oximetry, which shoots light through your skin to sense the heartbeat by the change in color of your skin based on your blood flow. When you use a watch (Apple Watch, Mio, Basis, etc), an ear clip (Heartmath), or a finger clip (as in those common in hospitals), they all use pulse oximetry.
This type of sensor is good for measuring heart rate, but NOT as good for Heart Rate Variability (HRV). This is because the way it measures heart beats will have a lot of errors when you move your body. There are medical studies on these error rates. Nothing wrong with your Apple Watch telling you your heart rate on a run or during the day. But if you are trying to get precise HRV information from that same watch while you are moving around you will have a problem. So you can use pulse oximetry if you plan to be motionless for your studies, but if you want to look at anything where you move your body you will need to use a different method.
The way to accurately measure your individual heart beats and then your HRV is based on reading the electrical impulses that make your heart beat. These sensors are heart rate belts or more expensive adhesive patches. These give you the required accuracy because they are reading the electrical output from your heart when it beats. I have tested pulse oximetry (Heartmath, Mio wrist band) vs Polar heart rate belt and the differences in accuracy when in motion are noticeable.
Find out more by reading about the Basic Equipment Setup.