1.   kernel is the main layer between operating system and hardware. Once the kernel is loaded into computer memory, the BIOS transfers control to the kernel. It then loads other OS components to complete the system startup and make control available to users through a desktop or other user interface. 

2.   No , Linux is not an operating system but a kernel. Linux is a GUI tools to control system. the appropriate name is Linux distribution. one of Linux distro is Ubuntu.

3.  No, its not completely true. Thus Linux is an open source , for this it is more secure than other operating system but it dose not mean that virus attack never happened. it is the structure of Linux that create difficulties for malware to take root access.