Found meaning of 'KV' after much digging, follow very carefully

OK, everyone follow very carefully, I shall type this only once. The meaning of KV as it is used in 'Allo 'Allo and You Rang, M'Lord? means "keep vigil" sometimes written "keep cavy". In other words "beware", "look out" for the "enemy", when a group is conducting a covert operation. This answer comes from the "Old Lawrencians" website under "Slang starting with K". Coincidentally, the French expression "sur le qui vive" also refers to heightened awareness. Literally it means "who lives?" or "who goes there?" as a military sentry would ask of intruders. However, applied in reverse, the intruders have to also be (quietly) on guard to avoid detection, as would any party up to some sort of mischief, and hence beseech its members to "K.V."