What kind of plastic or material are they made from?
Polyurethane, I believe.
Is it possible they are PVC?
Quite possibly, but I think those are less common. Many sellers will specify materials for allergy reasons, and PU seems to be the most common.
Polyurethane is most commonly referred to as ‘resin’ in watch bands, but silicone is sometimes called resin, and common plastic might be labeled as such.
A reputable manufacturer (e.g., Casio) that makes a bajillion resin watch bands a year is using polyurethane.
Additional info: Resin is more commonly used to define the watch case structure, so that a resin watch, such as Casio G-Shock series and other tough series they offer, will be touted as tough resin, and the strap is almost always a similar material to the case, except it is pliable and called polyurethane).
.