Her or His Majesty's Armed Vessel (HMAV) was a

ship prefix A ship prefix is a combination of letters, usually abbreviations, used in front of the name of a civilian or naval ship that has historically served numerous purposes, such as identifying the vessel's mode of propulsion, purpose, or ownership/n ...

formerly used for certain Royal Navy ships. The HMAV prefix now means

Her Majesty's Army Vessel His or Her Majesty's Army Vessel (HMAV) is the prefix used for a ship which is an operational unit of the British Army, commanded by British Army officers and crewed by army personnel in uniform. These ships fly the Army ensign.{{cite web , url=h ...

References

List of Acronyms Preceding the Name of a Ship
History of the Royal Navy Ship prefixes Year of disestablishment missing {{UK-mil-stub