Virtual memory is a concept, an illusion given to a process so it thinks it has large and almost infinite memory, and sometimes more than the system really has. It is up to the CPU to make the conversion from virtual to physical address every time you access a memory location. That mechanism is called address translation, and is performed by the Memory Management Unit (MMU), which is a part of the CPU.
MMU protects memory from unauthorized access. Given a process, any page that needs to be accessed must exist in one of the process VMAs, and thus must live in the process page table (every process has its own).
Memory is organized by chunks of fixed size named pages for virtual memory and frames for physical memory, sized 4 KB in our case. Anyway, you do not need to guess the page size of the system you write the driver for. It is defined and accessible...
