Bending arrows in tikz-cd

Maybe this?

% arara: pdflatex

\documentclass[a4paper,12pt]{amsart}
\usepackage{tikz-cd}

\begin{document}
\[
\begin{tikzcd}[row sep=-5pt, column sep=10pt,cells={nodes={draw, circle,font=\tiny, inner sep=2pt, minimum size=24pt}}]
     1 \arrow[dash]{dr} \arrow[dashed,bend left=25]{rrr} & &[.8cm] & 
     \ell-1 \arrow[dashed, rounded corners, to path={ -- ([xshift=-1.795cm]\tikztostart.west) |- (\tikztotarget)}]{llldd}
    \\
    & 2 \arrow[dash]{r}[description]{\ldots} & \ell-2 \arrow[dash]{ur}\arrow[dash]{dr} & 
    \\
    1 \arrow[dash]{ur} & & & 
    \ell \arrow[dashed,bend left=25]{lll} \arrow[dashed, rounded corners, to path={ -- ([yshift=-2ex]\tikztostart.south) -| ([xshift=-1.5ex]\tikztotarget.west) -- (\tikztotarget)}]{uulll}
\end{tikzcd}  
\]
\end{document}

I have not changed the arrow tips as the ones from xy are just ugly. If you want to change to some other style, just look into the manual. xy arrow heads are not included asaif.

You can try the dynkin-diagrams package. This example is included in the documentation. Note that by default, the affine root of this D series diagram is marked with a white circle, while the other roots have solid circles; you can easily change this if you don't like it.

\documentclass{amsart}
\usepackage{dynkin-diagrams}
\tikzset{big arrow/.style={
    -Stealth,line cap=round,line width=1mm,
    shorten <=1mm,shorten >=1mm}}
\newcommand\catholic[2]{\draw[big arrow,green!25!white] 
(root #1) to (root #2);}
\newcommand\protestant[2]{
\begin{scope}[transparency group, opacity=.25]
\draw[big arrow,orange] (root #1) to (root #2);
\end{scope}}
\begin{document}
\begin{dynkinDiagram}[edge length=1.2cm,
indefinite edge/.style={thick,loosely dotted},
labels*={0,1,2,3,\ell-3,\ell-2,\ell-1,\ell}]{D}[1]{}
\catholic{0}{6}\catholic{1}{7}
\protestant{7}{0}\protestant{6}{1}
\end{dynkinDiagram}
\end{document}