What is the difference between kernel mode and user mode?

Kernel mode (supervisor/privileged mode) allows unrestricted access to hardware, memory, and all CPU instructions. User mode restricts access to protect system stability - privileged instructions cause exceptions. Transition to kernel mode occurs via system calls, interrupts, or exceptions. This separation prevents user programs from crashing the system or accessing others' data. Hardware (CPU mode bit) enforces this protection. Hypervisor mode is even more privileged for virtualization.