Sixteenth notes are often neglected by many when improvising over a swing beat.
Observe how pianist David Newton demonstrates the use of sixteenth notes in his solo, at 3m33s of this video.
The basic rhythm for a swing beat is as shown in diagram 1 below
Thus an improvised melody maybe played like this over a swing beat
string of eight notes triplets fit nicely into swing beats too
A jazz improviser will usually apply a mixture of both in his solos, as shown in diagram 4 below
This alone is quite enough for a nice solo, however, an occasional use of sixteenth notes could always produce some interesting effect rhythmically
Here’s another example,
See how Diana Krall spice up her solo with sixteenth notes, at 1m23s of this video.
Students who have practised bebop eight notes soloing thoroughly should have no problem in this area. The sixteenth notes here could be treated as eight notes played in double time.
is equivalent to
However, keep in mind that due to this factor, applying sixteenth notes on a medium tempo swing beat of maybe 150bpm, would be like soloing over a bebop tune at 300bpm !!