last year my neighbour for C + M + B for a change.
That's actually the 'correct' inscription. The letters are the initial letters of the phrase 'Christus mansionem benedicat' 'Christ blesses this house/May Christ bless this house.
KMB is Kacper, Melchior and Balthazar.
That's the folk custom which grew out of a misconception of what the letters represented.
So you could say that C+M+B belongs to the church and K+M+B belongs to the people :)