\documentclass{article}
\usepackage{xepersian}
\settextfont[Scale=1]{XB Zar}
\setlatintextfont[Scale=1]{DejaVu Serif}
\usepackage{amsthm}
\theoremstyle{definition}
\newtheorem{defn}{تعریف}[section]
\newtheorem{thrm}[defn]{قضیه}
\begin{document}
\section{بخش یک}
\begin{defn}
\end{defn}
\begin{thrm}
\end{thrm}
\section{بخش دوم}
\begin{defn}
\end{defn}
\begin{thrm}
\end{thrm}
\end{document}