but as soon as I export it to KML file I can just see the polylines without the direction.
Is there a way to get the direction / Start and End points of travel in KML to open in Google Earth ?
The arrows you see in QGIS are a layer styling option in QGIS. As far as I can tell (not being an expert with Google Earth), Google Earth doesn't have a way to style a line as an arrow. Instead, you can export the start and end points as separate layers, then add them to Google Earth and display them with square and triangle icons.
Add 4 new fields using the
Field Calculator > Create a new field > type: Decimal Number
x( start_point( $geometry))
y( start_point( $geometry))
x( end_point( $geometry))
y( end_point( $geometry))
Right click on line layer in Layer panel > Save vector layer as... > CSV
Repeat for end points
Import each csv file created in step 2, using ..._point_x as the X field and ..._point_y as the Y field.
Right click on point layer in Layer panel > Save vector layer as... > KML