Hi Madila
HVAF (High Velocity Air Fuel) is similar to HVOF (High Velocity Oxygen Fuel). The flame temperature and heating effect on particles is somewhat lower with HVAF. This can have advantages with spraying materials like tungsten carbide based powders and the less refractory and ductile metal/alloys. Obviously it tends to less good with the more refractory higher melting type materials.
uniquecoat (link dead) will give you an idea of what its all about, bear in mind that it is a commercial website though