1. 

Kernel is a computer program that is a core or heart of an operating system. it has full control over everything in the system. Each operation of hardware and software is managed by the kernel. It acts as a bridge between applications and data processing done at the hardware level.It is the central component of an OS. It is the part of the OS that always resides in computer memory and enables the communication between software and hardware components

2.

Linux is not an operating system. It is a kernel. The Kernel is part of the operating system and the most crucial part. The technically correct term for this complete operating system is Linux distribution or simply Linux distro. Examples of popular Linux distributions include Ubuntu, Red Hat, and Debian.  

3. No operating system on this earth can be ever be 100% immune to Viruses and Malware. But still Linux never had a widespread malware-infection as compared to Windows. Though linux is not fully immune to virus, it gives a high level protection from virus.