1.   What is kernel?

The core and most important part of an operating system (OS) is the kernel. 
It is a piece of software that connects a computer system's hardware and software layers. 
The kernel is in charge of controlling system resources, offering critical services to programs, 
and making sure the operating system is stable and secure overall.


2.   Is Linux an operating system or a kernel?

Linux isn't an operating system; it's just a kernel, in theory. 
However, the term "Linux" is frequently used to refer to a whole operating system that comes with a shell (like bash)
and command-line and/or graphical user interface (GUI) facilities for system control.
This complete operating system is known as a Linux distribution, or simply a Linux distro, in technical jargon.

3.   Is Linux Operating System Immune to Malware?

An operating system can be assumed to be bug-free because it would be useless if it did.
I would answer "no" since computers are unable to differentiate between legitimate and dangerous software,
hence an anti-virus application is impossible.
A popular method for securing a system is to exclude programmers from using it and limit the software 
that can be run on it to that which has been authorized by management or the OS provider. 
Thus, the issue is reduced to blocking access to unauthorized programs. Here, a bug-free operating system without potential exploits helps, but people can still make mistakes or turn hostile.
A ROM in an embedded system might only contain one program, making it possible to change the software only
by physically changing the ROM. You wouldn't require an operating system in that scenario.
Perhaps a computer constructed using the Harvard architecture as opposed to the von Neumann design is more 
resistant to program modifications.