how to read and write xml file into string formate ?

Question

-<D2>
<INSTPARAM CODE="P1-2-1-1-0" UNIT="V" VALUE="64.56"/>
<INSTPARAM CODE="P2-1-1-1-0" UNIT="A" VALUE="0"/>
<INSTPARAM CODE="P1-2-2-1-0" UNIT="V" VALUE="64.74"/>
<INSTPARAM CODE="P2-1-2-1-0" UNIT="A" VALUE="0"/>
<INSTPARAM CODE="P1-2-3-1-0" UNIT="V" VALUE="63.68"/>
<INSTPARAM CODE="P2-1-3-1-0" UNIT="A" VALUE="0"/>
<INSTPARAM CODE="P4-1-1-0-0" UNIT="" VALUE="0"/>
<INSTPARAM CODE="P4-2-1-0-0" UNIT="" VALUE="0"/>
<INSTPARAM CODE="P4-3-1-0-0" UNIT="" VALUE="0"/>
<INSTPARAM CODE="P4-4-1-0-0" UNIT="" VALUE="0"/>
<INSTPARAM CODE="P9-1-0-0-0" UNIT="Hz" VALUE="50.02"/>
<INSTPARAM CODE="P3-4-4-1-0" UNIT="k" VALUE="0"/>
<INSTPARAM CODE="P3-2-4-1-0" UNIT="k" VALUE="0"/>
<INSTPARAM CODE="P3-3-4-1-0" UNIT="k" VALUE="0"/>
<INSTPARAM CODE="P7-1-5-2-0" UNIT="k" VALUE="20.29"/>
<INSTPARAM CODE="P7-2-1-0-0" UNIT="k" VALUE="77.23"/>
<INSTPARAM CODE="P7-2-4-0-0" UNIT="k" VALUE="0.1"/>
<INSTPARAM CODE="P7-3-5-2-0" UNIT="k" VALUE="80.58"/>
</D2>