A ramdisk is a virtual disk entirely composed of RAM. This means that all data on the virtual disk is located in the memory, thus it greatly decreases access time and achieves the best performance.
A hybrid-disk is a virtual disk composed of RAM and physical storage. It is introduced because usually a ramdisk, compared with physical storage, is small capacity and sometimes cannot meet storage requirement. The hybrid-disk firstly store data to the memory. When all of the memory has been used up, it begins to store data to a file which is located in a physical drive.
A file-disk is a virtual disk entirely composed of physical storage. All access requests to the virtual disk are transferred to a file located in the physical drive. Because the physical drive is a non-volatile storage, all data on the file-disk is preserved across system restarts, without need of the imaging request.
Only ramdisk supports setting the system paging file on it, neither hybrid-disk nor file-disk supports it.