Although it is in many breeds and clubs, crossbreeding is not absolutely forbidden in ALL dog breeding and breed clubs. These clubs still do call their dogs purebreds, so that definition is not set in stone as being pureblood by ancestry.
The Jack Russel Terrier Club of America allows in first generation crosses, as long as they have passed evaluation, as do some of the American Border Collie clubs and registries. I'm sure there are others.
I'm in no way suggesting, at this time, the crossbreeding of Cavaliers, not when so much has been put into the EBVs, but wanted to point this out.