But Warwickshire's England left-arm spinner Ashley Giles checked the progress of Kent, runners-up last season, with five for 82 including the wicket of van Jaarsveld who struck 15 fours.
It was another impressive haul from Giles who last week took nine wickets in the win over Glamorgan.
Earlier England batsman Robert Key fell for four when he was caught by Jonathan Trott off former Zimbabwe captain and pace bowler Heath Streak
Van Jaarsveld and Darren Stevens shared a stand of 96 before the South African fell.
At Lord's, Australian batsman David Hussey's unbeaten 117 helped Nottinghamshire to 399 for six against Middlesex.
Elsewhere in the top flight Hampshire seamer Chris Tremlett took six for 44 as Sussex were bowled out for 252 with Michael Yardy following his hundred against Surrey with 104.
In the Second Division Worcestershire were bowled out for 171 by Durham with the hosts Mark Davies taking six for 32 at the Riverside.
Durham were 139 for two at stumps with England all-rounder Paul Collingwood 88 not out.
In the little play that was possible at Wantage Road, Northamptonshire reached 90 without loss against Leicestershire while the whole of the first day was washed out in the game between Yorkshire and Somerset at Headingley.