Virtual memory allows the execution of processes that may not be completely in the main physical memory. It uses both hardware and software to enable a computer to compensate for physical memory shortages by temporarily transferring data from RAM to disk storage.

Deadlock can be handled using various strategies, such as prevention, avoidance, detection, and recovery. One common method is to use algorithms like Banker's algorithm to avoid deadlock, or to impose resource ordering to prevent it.

Scheduling algorithms include First-Come-First-Serve (FCFS), Shortest Job Next (SJN), Priority Scheduling, Round Robin (RR), and Multilevel Queue Scheduling. Each has its own advantages and is chosen based on the specific requirements of the task.

A process is an independent program in execution with its own address space, while a thread is a smaller unit of a process that shares the same address space with other threads within the same process. Threads allow parallelism within a process at a finer granularity.

Operating systems use several mechanisms such as user authentication, access control policies, encryption, auditing, and secure coding practices to ensure security. They also employ measures like firewalls, antivirus programs, and regular system updates to protect user data from vulnerabilities and attacks.

Monolithic kernels have all the operating system services running in the kernel space, which can be efficient but may lead to less modularity and greater risk if a service fails. In contrast, microkernel architectures run most services in user space, with the kernel managing basic operations. This leads to better modularity and stability, though it can involve more context switching and overhead.