Voltage drop when connecting DC-DC booster XL6009 to my solar panel generating 10V constant
That's normal behavior for a solar panel, when you try to draw more current than it's capable of delivering.
The maximum current is determined by the size of the panel and the amount of light falling on it.  If you draw less than that current, then the voltage is fairly stable.  Draw too much, and the voltage collapses.
You need more light or a bigger panel.  Or re-design your circuit to run at a lower current.