A DVD is standard definition by specification. HD video is 1920 x 1080 or 1280 x 720 at minimum. HD video must be down-converted to SD video (720x480 see What is Anamorphic?) before it can be authored to a DVD. As a part of the down conversion process, the video resolution is reduced to accommodate the SD format. By specification SD video is a lower resolution and will result in quality loss. How much quality loss will depend on original shooting format, CODEC, and the software or hardware used to down convert the video. For instance, a video shot on a high end Alexa or Red Dragon camera will yield better results because it is uncompressed vs. a Canon 5D or 7D which uses a compressed Mpeg4 format.
