Difference bewteen SvPV_nolen() and SvPVbyte_nolen(), and how to get SvPVbyte_nolen() used?

[kyfung]

Probably a newbie question. I had to modify a
SvPV_nolen() to SvPVbyte_nolen() to get some code to
work properly.

What is the difference between SvPV_nolen() and
SvPVbyte_nolen()? How do I get SvPVbyte_nolen() used
for  a parameter instead of SvPV_nolen()?

If the type of a parameter is T_PV in typemap, I get
SvPV_nolen() for the parameter. What type should I use
to get SvPVbyte_nolen() instead?

Thanks.