purchase_discount/views/report_purchaseorder.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <template id="report_purchaseorder_document" inherit_id="purchase.report_purchaseorder_document">
            <xpath expr="//th[strong[text()='Net Price']]" position="before">
                <th class="text-right"><strong>Disc. (%)</strong></th>
            </xpath>
            <xpath expr="//td[span[@t-field='line.price_subtotal']]" position="before">
                <td class="text-right">
                    <span t-field="line.discount"/>
                </td>
            </xpath>
        </template>
    </data>
</openerp>