MythTV requires tuner drivers which support the V4L standard.
Currently no official FreeBSD driver does so, but the latest version
of a beta version of the cxm driver is available from
http://usleepless.110mb.com/pvrxxx_port-10042007.tgz. Problems
have been reported with this port]
There is a one more site, which you should read. This is a guide written by Greg 'Groggy' Lehey , who is also the author of fantastic book about FreeBSD.
MythTV on FreeBSD HOWTO.
I hope that it will be useful for you.
By the way, you can also look here;
multimedia/v4l_compat,
multimedia/libv4l,
multimedia/pvrxxx and
multimedia/pvr250.
V4L support for FreeBSD.