\documentclass{article}
\usepackage[all]{xy}
\usepackage{tikz}
\begin{document}
\[ \xymatrixrowsep{50pt}\xymatrixcolsep{50pt}\xymatrix{
 P' \ar@/^1pc/@{.>}[rrd]^{\tilde g} \ar@/_1pc/@{.>}[ddr]_{\tilde f}
 \ar@{.>}[rd]^h \\
   & P \ar[r]^{g'} \ar[d]_{f'} & X \ar[d]^f \\
     & Y \ar[r]_g \ar@{}[ur]^(.3){}="a"^(.7){}="b" \ar@{=>}_\eta "a";"b" & Z
} \]
\end{document}
