Answer - 1:
Kernel is the heart of all operating system. It provides the interface to the hardware devices as well as processes also. It controls all the major hardware works related to OS to perform an operation.

Answer - 2:
 Linux is just a kernel. It's not an operating system.  Kernel is a software which is included in every OS.

Answer - 3:
 Yes, Linux software is often immune to malware.  Linux need an antivirus protection for real. But it's known to everyone that Linux is a secured OS. But it can be attacked by malware. So Linux is immune to malware and it needs an antivirus for protection for all necessary files.