As the name shows, a Memory Resident Virus is a type of infection that enters and resides in your system's memory. As soon as it reaches there, your antivirus application is then unable to influence it.
Polymorphic is a subtype of File Infector Virus, infecting files and folders. It has the potential to contaminate your data by writing certain malicious codes. Just like its descriptive name, it holds a continuously changing behavior.