Four other guards were wounded in the attack in the country's volatile south.
General Zafaruddin, the Wardak police chief, said two policemen who went to help the guards were also killed in the ambush which he described as a new Taliban tactic.
"They jumped from their vehicle and opened fire on Ainullah's vehicle – he was in charge of the private security guards," he said on Sunday.
Most violent year
This year has been Afghanistan's most violent since the 2001 US-led invasion drove the Taliban from power.
More than 6,300 people, mostly militants, have been killed in insurgency-related violence, according to an Associated Press count.
In a separate incident on Sunday a roadside bomb attack killed a Nato soldier and left four others wounded in southern Afghanistan.
The Nato-led International Security Assistance Force (ISAF) said in a statement the soldiers were carrying out a routine vehicle mounted patrol when the bomb went off.
The ISAF, made up of troops from more than three dozen countries, is helping the Afghan government to secure the country in the face of ongoing attacks from Taliban and other fighters.
This year, nearly 220 soldiers with ISAF or a separate US-led coalition have been killed in Afghanistan mostly in hostile action, according to an AFP count based on official reports.