Magnum simply refers to a larger cartridge case, as a magnum of champagne is a bottle twice the standard size.
Had the same naming tradition been followed, the RUM cartridges should have been called Jeroboams, and the big Weatherbees perhaps Rehoboams.
Hypervelocity should refer to velocities in excess of mach 4, or roughly 4,000 fps.
However both 'magnum' and 'hypervelocity' are marketing terms, so they really only mean what the marketing team decides they mean.
So to answer the question, "What's the difference?" the answer is "Lots." or maybe "Nothing", or something in between.
"Those who have long enjoyed privileges as we enjoy, forget, in time, that men have died to win them."
Franklin D. Roosvelt