You'll need to define a support point for this. You can use the syntax +(<x>,<y>)
, which specifies a point relative to the starting point. In order to smooth the curve, you'll have to make sure that the in
value of the path segment going into the support point, and the out
value of the segment leaving the support point add up to 0. To adjust the path of the curve, you can play with looseness
, in looseness
and out looseness
, which specify how "curvy" the path is.
By specifying an out
and in
angle for the first and last segment of the path, you can determine at what location the path leaves or enters a node. Alternatively, you could specify (<node name>.north west)
to make the path start or end at the top left of a node, or use (<node name>.<angle>)
to specify at what angle on the shape outline the path will start or end, with 0
pointing due east, 90
due north, etc.

\documentclass[a4paper]{article}
\usepackage{tikz}
\usetikzlibrary{shapes.geometric,positioning}
\begin{document}
\begin{tikzpicture}[every node/.style={draw,ellipse}]
\node (A) {A};
\node [below left=2cm and 0.5cm of A] (B) {B};
\draw[out=-90,in=90,->] (A) to (B);
\draw [->] (B) to [out=-90,in=-90,in looseness=2.5] +(-1cm,1cm) to [out=90,in=135,out looseness=1.5](A);
\end{tikzpicture}
\end{document}
If you're not set on the path leaving node B
from the south, but instead make it leave from south west, you can instead just write
\draw [->] (B) to [out=-135,in=135,in looseness=1.5] (A);
