Difference: Different battery life.
Huawei P30 is equipped with 3650mAh battery, supporting 22.5W fast charge; The Huawei P40 is equipped with 3800mAh battery and supports 22.5W fast charge.
Huawei P30 is equipped with Kirin 980 chip and does not support 5G network. Huawei P40 is equipped with Kirin 990 5G chip, which supports 5G full Netcom.