If you by 'use proxy in xterm' mean when using e.g. curl or wget, then it will need an environment variable set. Ideally the proxy setting should be exported to the environment automatically but that doesn't seem to be the case for xterm

This works for me, when I use privoxy as integrated proxy. Replace 127.0.0.1 and/or 8118 with IP/name:port you use for your proxy:

export http_proxy=http://127.0.0.1:8118
If you need an ftp proxy as well then use the exact same (http://) value for ftp_proxy.