8mm film has about 800 to 1000 lines of resolution. DVD has 480 lines. A HD scan of your 8mm film will capture 100% of the details from the film. An 8mm film to DVD transfer will only capture about 60% of the details from your film.
But, there are different ways to perform an 8mm film HD transfer which can make a huge difference. There is a real-time HD process, a frame by frame HD process and a film scanner HD process. A real-time HD film transfer process uses a HD camcorder and records your film as it goes by in real-time (the same speed it would play on a projector). Because it uses a camcorder in a real-time fashion, this process will produce video that is about 40-50% worse than the film itself.
A frame by frame HD process still uses a HD camcorder but the equipment stops at each frame and the camcorder is used to take a picture of the frame. This produces slightly better quality than a real-time HD film transfer process.
The best process is an HD film scanning process. The film is scanned using a professional broadcast quality film scanner used by the movie studios. An HD film scanner will produce a video is that about 30 to 50% better quality than an HD frame by frame process.