On my LCD HDTV, regular TV through component cables from the cable box looks very grainy (looks slightly blurry through a TV cable), but HD broadcasts and DVDs--including those from the HC3, iMovie and iDVD--are much better. If you're talking about outlines around objects, that could be from over-sharpening. You can adjust this in the sharpening item in the camera set of the camera/tape menu. Things that are out of focus should just appear soft, not grainy. If you're viewing the image directly from component or HDMI from the camera, it will be slightly softer than true HDTV, though better than DVD, because HDV is 1440, not 1920, pixels wide, but I wouldn't expect that should make it grainy.
I presume you're importing it into iMovie as an HDV project. If you're editing them as regular DV projects and viewing them from DVDs on a large screen TV, that could make your HC3 video look chunky and pixelated.
Hope that helps.