I guess it's my day to be contrarian.

xv is a great program, but the last revision was 3.10a, which is now eight (8) years old (though there have been patches since then for a new format or two).

I use ImageMagick, which can also do the rectangular captures and do most or all of the image processing you mention.

If you just want a whole screen dump, try xwd, which I believe is in the base X11 distribution. You can then do your image processing/conversion with xv, ImageMagick (components of which are scriptable, BTW), Gimp or whateversaround. Your laundry list of requirements looks more interactive, so postprocessing with Gimp may be your best choice.

As for capturing screen changes over time, if xwd in a shell script loop doesn't meet your needs (it is pretty limited), I'm at a loss.