That means that Pompey remain rooted to bottom spot, while Gianfranco Zola can, hopefully, look onwards and upwards as his side head to Tottenham Hotspur on Monday.
Buoyed by the surprising 1-1 draw with table-topping Chelsea last weekend, the Italian made just one enforced switch as James Tomkins came in for Danny Gabbidon (hamstring).
But on 16 minutes, Zola was forced to replace Mark Noble (hamstring) with Luis Jimenez and, after referee Lee Probert had booked both Aruna Dindane and Michael Brown in quick succession for unseasonal assaults on Scott Parker, the Chile international was the next man to feel the full force of a Pompey challenge.
This time, though, skipper Brown's reckless 23rd-minute trip on Jimenez was to prove costly, for it was inside the Pompey area and, cool as you like, Diamanti sent the spot-kick wide of the diving Asmir Begovic to claim his fifth goal of the season.
Despite their shock win over Liverpool last weekend, West Ham's dogged draw had meant that Portsmouth still found themselves in dreaded bottom spot on Christmas Day and there was to be nothing, but cold turkey for Avram Grant's team here at Upton Park.
The Pompey boss had made two changes from the side that had rattled Rafa's Reds, as ex-Hammer Hayden Mullins and Dindane replaced substitutes Kevin-Prince Boateng (virus) and Hassan Yebda, but apart from Jamie O'Hara's 25-yard free-kick, which was easily fielded by Robert Green and a wayward Tal Ben Haim effort, the rock-bottom visitors hardly looked like clawing themselves level before the break.
At the other end, Jack Collison had spurned two great chances to double the Hammers' lead before the break and, just after the restart, Begovic brilliantly palmed away Diamanti's 18-yarder after it took an awkward, looping deflection off Steve Finnan.
With Kevin-Prince Boateng already on for Nadir Belhadj, Kanu then joined the action as the delinquent Dindane was taken off before he was sent off by referee Probert , whose patience had been tested to the limit by the Ivory Coast international.
On the hour, Green bravely dived into the studs of Frederic Piquionne and then Boateng slid into the sidenetting under pressure from Herita Ilunga after the Hammers' keeper had misjudged the bounce of the ball.
As Pompey desperately tried to get themselves back into the contest, O'Hara became their third booking of the day for a tackle on Kovac, before Piquionne blasted over from a tight angle.
Diamanti was then given a standing ovation for a diamond-like display, when he made way for Valon Behrami for a final 20 minutes that saw Boateng scuff to Green, before Piquionne's low 15-yarder was brilliantly held by the Hammers' keeper.
But after Collison and Tomkins went close, Kovac made sure of victory when he met Jimenez's inch-perfect free-kick with a powerful header.