When the locals tunnel into the POW camp

Recently started the box set. Have seen most of the series on PBS a while back. In the eps where they tunnel into the POW camp so that the airmen can be supplied with uniforms (to avoid, when captured, being shot as spies) The British insiders use the term "KV" as part of their dialogue. At least that is what it sounds like, and is spelled out in the subtitles as such. So what does it mean? Not that it deters much from enjoying the show. Have not been able to find a likely definition among the dozens of listings (or dizens of loostings as Crabtree would say it)on the internet. Had to be a great show to have generated so many favorable comments - there will always be dissenters, or dissers. Helga (Kim Hartman) had a gorgeous body and did a red-hot strip tease in the ep. "Knockwurst in the Cupboard". René (Gorden Kaye) kept a straight face through all nine series. Most of the actors showed great comedy talent, and must have enjoyed their job. Any ideas for KV, please?