EvilZone

Please give me a detailed answer.  I'm not trying to write any malware (even if I wanted to, I never would as I clearly don't understand it fully), I'm just interested in computer security and I've don't get how stuff hides from the antivirus on a technical level.  Everywhere I look for an explanation, it never actually explains the technical stuff, and it always just says something like "it hides from the antivirus".  Could anyone here explain some typical techniques used by malware to hide from antivirus software?  Looking for stuff like working with memory in a way that it goes undetected or installing to a certain location in the filesystem (those probably aren't actual methods, but just as examples of the technical description I'm looking for).  Also, how does malware sometimes kill antivirus software?

Thanks for your time.