HDTV means High Definition. It's a TV that has a higher DPI than normal TV, resulting in a far sharper image.
DTV however, is not a television.
DTV is the means by which the signal is delivered to your house. If you have cable, then that's
DTV. If you use an antenna, then that's ATV, or Analog Television.
As for what DPI means, it means Dots Per Inch. Basically, how many pixels there are on the television screen. The higher it is, the sharper the image.