Salva scored in the 22nd and 34th minutes at the Estadio Benito Villamarin - adding to Jorge Molina's ninth-minute opener - as Valencia were blown away.
It was a woeful first-half showing from Los Che, whose coach Miroslav Djukic is finding life in charge of a side shorn of its stars a struggle.
With Roberto Soldado at Tottenham and David Albelda retired, Valencia are struggling to make headway this season and have just three points, with Ricardo Costa's goal their only notable moment in this clash.
They had actually started brightly - Jonas heading wide - which made what followed so surprising.
With nine minutes on the clock, Salva rolled a simple ball into the feet of Molina and unchallenged he drilled a low, raking effort from the edge of the area beyond Diego Alves.
It was a well-taken goal from Molina but nothing compared to Salva's brilliant first.
He started the move midway through the Valencia half and, in the blink of an eye, passes were threaded by Juanfran and Molina, leaving Salvo clean through and with the time to slot beyond Alves.
The midfielder was enjoying the run of the field and his second soon arrived, with Alvaro Vadillo's brilliant no-look pass putting him clear on the left and he tucked in a third.
He worked Alves before the break too as he sought out a first-half treble, while better accuracy from Molina could have made it four when the keeper failed to fully deal with Salva's free-kick.
Javier Matilla also stung the palms of Alves and another goal looked inevitable, although when it arrived - Costa heading in Ever Banega's cross - it was not the strike many expected.
Cedrick got Betis back on the front foot and drilled a shot at Alves and the striker, such a livewire since joining from Numancia for little over a thousand euros, then cracked an effort just over.
There would be no further goals, though.