Suppose the earth revolves around the sun in a circular orbit,...

In uniform circular motion, the instantaneous acceleration is the centripetal acceleration, which is always directed towards the centre of the circular path. Since the earth revolves around the sun in a circular orbit, its instantaneous acceleration is directed towards the sun.

From 1st Jan to 30th June (approximately half a year), the earth completes half of its orbit. The displacement is equal to the diameter of the orbit, which is non-zero, making the average velocity non-zero.

During this half-year period, the change in velocity is $2v = 60 \text{ km/s}$. The average acceleration is $\dfrac{\Delta v}{\Delta t} = \dfrac{60}{\text{half a year in seconds}} \text{ km/s}^2$, which is much less than $60 \text{ km/s}^2$.

From 1st Jan to 31st Dec (one full year), the distance travelled is the circumference of the orbit. The average speed is the constant speed of $30 \text{ km/s}$, not zero.