Hello Harshit,

In odoo 11 you can use the origin field on the purchase order.

Example: The SO11 creates a purchase order with code PO33 and field "origin" with the SO11 value.

You can override the cancel method on the sale order like this:

class SaleOrder(models.Model):
    _inherit='sale.order'

    @api.multi
    def action_cancel(self):
        purchase_order = self.env['purchase.order'].search([('origin','=',self.name)])
        if purchase_order:
            purchase_order.button_cancel()
        res = super(SaleOrder, self).action_cancel()
        return res

Hope it helps.