The CMU fonts are based on Computer Modern (and the CB fonts for Greek):
\documentclass{scrreprt}
\usepackage{fontspec}
\setmainfont{CMU Serif}
\setsansfont{CMU Sans Serif}
\newfontfamily{\greekfont}{CMU Serif}
\newfontfamily{\greekfontsf}{CMU Sans Serif}
\usepackage{polyglossia}
\setdefaultlanguage{greek}
\begin{document}
\tableofcontents{}
\section{Αυτό είναι ελληνικά.}
{\bfseries Welcome to Greek:} Αυτό είναι ελληνικά
Or Russian: Привет
%Or Hebrew: מה הבעיה?
\end{document}
There's no support for Hebrew, though.

You can support Hebrew with a different font and using ucharclasses:
\documentclass{scrreprt}
\usepackage{fontspec}
\setmainfont{CMU Serif}
\setsansfont{CMU Sans Serif}
\newfontfamily{\greekfont}{CMU Serif}
\newfontfamily{\greekfontsf}{CMU Sans Serif}
\newfontfamily{\hebrewfont}{Linux Libertine O}[Scale=MatchUppercase]
\usepackage{polyglossia}
\setdefaultlanguage{greek}
\usepackage[Latin,Greek,Hebrew,Cyrillic]{ucharclasses}
\setTransitionsFor{Hebrew}{\begingroup\hebrewfont}{\endgroup}
\begin{document}
\tableofcontents{}
\section{Αυτό είναι ελληνικά.}
{\bfseries Welcome to Greek:} Αυτό είναι ελληνικά
Or Russian: Привет
Or Hebrew: מה הבעיה?
\end{document}

xelatexto compile, then the simplest thing is to select a main font that includes the Unicode ranges you need. In this case thelibertinepackage will get you all these ranges without any further setup.If you select a mainfont that does not include the other ranges (e.g., Latin Modern), then you have to manually set up font families for the other languages. But then you will have different fonts for each set of characters that will not necessarily harmonize.
– musarithmia Feb 17 '15 at 19:00