What exact model TV is it? Sometimes the TV settings are the culprit, not the computer. Many TV's also have one specific HDMI input that is configured for computers. We need to find your manual online if you don't have it and see what inputs/settings the manufacturer recommends and see if that fixes the issue.
If you can't find the manual, have the Mac on, go into the HDTV's settings menu and change the picture size, if it has that option. Most have Zoom, Widescreen, etc. Just keep selecting them until you find the one that works. But also, read the manual because many HDTV's have a specific HDMI input that is best configured for PC's.