\documentclass[12pt]{article}

\usepackage[a5paper,margin=1cm,nohead,includefoot]{geometry}
\usepackage{indentfirst}
\usepackage{fontspec,xunicode,xltxtra}
\usepackage[russian,english]{polyglossia}
\defaultfontfeatures{Mapping=tex-text, Scale=MatchLowercase}
\setmainfont{Times New Roman}
\SetSymbolFont{letters}{normal}{EU1}{\rmdefault}{m}{it}
\usepackage{amsmath}                                                       
\usepackage{xecyr}
\renewcommand{\alpha}{α}
\renewcommand{\beta}{β}
\renewcommand{\gamma}{γ}
\renewcommand{\delta}{δ}
\renewcommand{\epsilon}{ε}
\renewcommand{\varepsilon}{\textit{ε}}
\renewcommand{\zeta}{ζ}
\renewcommand{\eta}{η}
\renewcommand{\theta}{θ}
\renewcommand{\vartheta}{\textit{θ}}
\renewcommand{\iota}{ι}
\renewcommand{\kappa}{κ}
\renewcommand{\lambda}{λ}
\renewcommand{\mu}{μ}
\renewcommand{\nu}{ν}
\renewcommand{\xi}{ξ}
\renewcommand{\pi}{π}
\renewcommand{\varpi}{\textit{π}}
\renewcommand{\rho}{ρ}
\renewcommand{\varrho}{\textit{ρ}}
\renewcommand{\sigma}{σ}
\renewcommand{\varsigma}{\textit{σ}}
\renewcommand{\tau}{τ}
\renewcommand{\upsilon}{υ}
\renewcommand{\phi}{φ}
\renewcommand{\varphi}{\textit{φ}}
\renewcommand{\chi}{χ}
\renewcommand{\psi}{ψ}
\renewcommand{\omega}{ω}
\renewcommand{\partial}{∂}
\begin{document}
Testing XeLaTeX!
Greek: τεχ.
$А_б^с$
$\lambda_{\text{в}},.:;$
$K_{с.вн}$
\end{document}