some hints for compiling Xen 3.1:

if you wan't the VNC/SDL feature with HVM you need the following packages:

 libjpeg62-dev, libvncserver-dev,  libsdl1.2-dev

and you need to set the following var:

export XENFB_TOOLS=y

and maybe you miss hvmloader: you need the package bcc or it will just not compile.

To check build dependecies there is a nice script:

cd tools/check

./chk build

- Thomas 

PS: see also: http://wiki.xensource.com/xenwiki/BuildConsiderations