It could also be due to limitations in your machine and/or monitor. Machines with less than a 350mhz processor have an option in iMovie preferences for smooth/blurry playback or jerky/sharp playback. They aren't powerful enough to provide both smooth and sharp playback.
Plus, as scott said, if you blow a DV up to full-screen on a computer monitor it'll look grainier. But if you play it back on a TV it'll look fine.
QuickTime movies are compressed and/or have lower frame rates so the video usually looks even worse than in iMovie.
DV resolution is actually 720x480 NTSC (720x576 for PAL). iMovie and TV screens artificially make it appear at the 4:3 ratio of 640x480. But if you export to a full-size, uncompressed QT movie, it'll be at the actual 720x480 NTSC (720x576 PAL) resolution. That might explain the different resolution numbers you saw in QT.