SVOD = Subscription VOD
Consumer pays a subscription fee (usually by a monthly rate) for watching with no limits.
The prime example is Netflix.
TVOD = Transactional VOD
Consumer. pays each time for the content they choose to watch. Usually there are two options - rent and buy.
The prime example is FandangoNOW.
AVOD = Advertising VOD
Consumer is allowed to watch content for free with advertisements placed throughout the watch period.
The prime example is Tubi TV.
Many streaming services have a combination of two or even all three of SVOD, TVOD and AVOD models.