As far as I can see you are asking to change the font in komacv
from sans serif to roman, right?
In chapter 3.7 Encoding and Fonts of the komacv
documentation (call texdoc komacv
on your terminal!) you can read that the style for a section is defined like:
\setkomafont{section}{\Large\sffamily\mdseries\slshape}
Just change it to
\setkomafont{section}{\Large\rmfamily\mdseries\slshape}
And to get the roman font in normal text you need to add
\renewcommand\familydefault{\rmdefault}\normalfont
at the beginning of your document.
So with the following complete MWE
\documentclass{komacv}
\usepackage{lmodern}
\setkomafont{section}{\Large\rmfamily\mdseries\slshape} % <=============
\begin{document}
\renewcommand\familydefault{\rmdefault}\normalfont % <==================
\section{M}
\cvitem{M}{M}
\end{document}
you get the result:
Do not forget to change the other font definitions to use \rmdefault
...