In common (everyday) speech you are using the two-word version, "never mind," which means to disregard or ignore. Don't use the outdated "nevermind" because, I promise you, you will use it wrong. In its correct usage, it doesn't sound like correct English.
Never mind what Tom says, he is crazy!
Never mind the manager, he is upset with everybody every day.
For what it is worth, "nvm" is NOT a word.