Other way around.

An acronym is a type of initialism, which is itself a type of abbreviation.

So acronyms are initialisms where you pronounce the letters like a word (e.g., RAM), initialisms are abbreviations made by taking the initial letters of multiple words and concatenating them regardless of how it's spoken (e.g. FBI for Federal Bureau of Investigation), and an abbreviation is any shortening of a word or phrase into something shorter (e.g., "abbrev." for abbreviation or "US" for United States).