This is the Series of Stanza that you will need to send to get Archived Messages. For more detail you can checkout http://xmpp.org/extensions/xep-0136.html
REQ
<iq type='get' id='mrug_sender@staging.openfire.com'>
<list xmlns='urn:xmpp:archive'
with='mrug_target_155@staging.openfire.com'>
<set xmlns='http://jabber.org/protocol/rsm'>
<max>6900</max>
</set>
</list>
</iq>
RES
<iq type="result" id="mrug_sender@staging.openfire.com" to="mrug_sender@staging.openfire.com/Psi">
<list xmlns="urn:xmpp:archive">
<chat with="mrug_target_155@staging.openfire.com" start="2014-06-07T06:52:26.041Z"/>
<chat with="mrug_target_155@staging.openfire.com" start="2014-06-07T07:06:53.372Z"/>
<set xmlns="http://jabber.org/protocol/rsm">
<first index="0">866</first>
<last>867</last>
<count>2</count>
</set>
</list>
</iq>
REQ
<iq type='get' id='mrug_sender@staging.openfire.com'>
<retrieve xmlns='urn:xmpp:archive' with='mrug_target_155@staging.openfire.com' start='2014-06-07T06:52:26.041Z'>
<set xmlns='http://jabber.org/protocol/rsm'>
<max>8000</max>
</set>
</retrieve>
</iq>
RES
<iq type="result" id="mrug_sender@staging.openfire.com" to="mrug_sender@staging.openfire.com/Psi">
<chat xmlns="urn:xmpp:archive" with="mrug_target_155@staging.openfire.com" start="2014-06-07T06:52:26.041Z">
<from secs="0" jid="mrug_target_155@staging.openfire.com">
<body>Wow !! This is Archived Message</body>
</from>
<set xmlns="http://jabber.org/protocol/rsm">
<first index="0">0</first>
<last>0</last>
<count>1</count>
</set>
</chat>
</iq>
To Fetch List of all Conversations
<iq type='get' id='mrug_sender@staging.openfire.com'>
<list xmlns='urn:xmpp:archive'>
<set xmlns='http://jabber.org/protocol/rsm'>
<max>6900</max>
</set>
</list>
</iq>