On i386: 
aptitude install ubuntu-xen-server

on amd64:
aptitude install linux-image-xen bridge-utils libxen3.1 python-xen-3.1 xen-docs-3.1 xen-hypervisor-3.1 xen-ioemu-3.1 xen-tools xen-utils-3.1


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vi /etc/xen/xend-config.sxp

#COMMENT (insert #) on the line so that it looks like:
#(network-script network-dummy)

# UNcomment (remove "#") so that it looks like this:
(network-script network-bridge)
(vif-script vif-bridge)