Again, I’m going to suggest - no, beg - that you try to write as simply as you can. Simple writing is not easy: it’s a real challenge to express complicated ideas in the simplest language possible. Don’t think that IELTS examiners will penalize an essay written in a simple, easy-to-read style, or that they prefer complicated vocabulary and structures. Simple, clear, and error-free is good. 

Introduction

This need for simple clear error-free writing is particularly important in your intro. The first word ('Discrepancy') in your intro is complicated, low frequency -  and wrong. It’s missing an article. The reference is unclear.  This is the first word of your essay  - it must be correct! Would a simpler choice have been easier?

You wrote:

Discrepancy between developed and under developing countries has been steadily increasing in recent years. In particular facilities as well as quality of academic courses are incomparable.Therefore an enormous number of talent students opts to leave their countries and study overseas, seeking for better opportunities. this trend involves concurrent benefits and drawbacks.

Here’s a suggested revision.

The gap [in living standards/wealth/resources] between developed and developing countries has been steadily increasing in recent years.

But the problem is that this is still not relevant to the question topic. Let’s see can we relate it to the topic

The gap between the quality of education in developed and developing countries has been increasing in recent years.

OK, now it’s relevant  to the question, not off-topic. [But is it true? Many universities and colleges from developing countries have entered the top 100 worldwide (see the annual USNews list of top universities), pushing long-established Western schools far down the list.] The problem is the phrase “increasing in recent years” – probably one of the most over-used phrases in IELTS introductions.

No matter what the topic, IELTS candidates love to say that it is "increasing in recent years" -  divorce, wife beating, calls for the death penalty, topiary, animal husbandry, steam engines -  you name it!

There are some other problems with the intro. Where are the facilities 'incomparable'? The reference is unclear -  it could be in either developed or developing countries. The last sentence, your thesis sentence, could be used in any essay. It needs to be specific to the topic and it needs to tell the reader what you are going to do. It can also give your opinion.

Here’s a possible rewrite of your introduction.

Because many developed countries have better educational facilities and courses, many talented students from developing countries opt to study overseas. In this essay, I will outline some of the benefits and difficulties of this for students and for the developing country.

Paragraph Two

I like your introduction to Paragraph Two:

On the one hand, consider a gifted person in his twenties in Bangladesh

However, the “on the one hand” phrase leads the the reader to expect a parallel introduction to Paragraph Three. It's disappointing when it doesn't happen. Let’s remove the phrase to leave an interesting sentence:

Let’s consider a gifted person in his twenties in Bangladesh. 

Your next sentence was:

Whereby, corrupted social and political systems as well as widespread poverty have created an inefficient educational system

This is a fragment. It’s not a complete sentence. We can join it to the previous sentence or add/delete some information to make it self-sufficient. Let’s delete the semi-archaic word ‘whereby’

Corrupt social and political systems as well as widespread poverty have created an inefficient educational system. 

Meanwhile those untapped potential could be unlocked at Sweden universities. ==> Meanwhile that untapped potential could be unlocked at Swedish universities, which  provide moderately-priced, productive modern learning methods and materials.

On the other hand, this process can be negatively affected if people do not return to home country or fail to complete their education.

Which process? It’s a new paragraph.

Articles

• affected if people do not return to home country ==>  If people do not return to their home country
• Discrepancy between developed and under developing countries ==>The discrepancy
• It will play a pivotal role in prosperity of poor countries è It will play a pivotal role in the prosperity of poor countries

Word Choice

Ultimately, the host country's investment will be lost in this case.==> [Here the host is the country hosting the overseas student, e.g. Sweden. Just omit -  it’s clear enough without it.]

For Instance, cultural and lingual differences, the intrinsic difficulty of academic education , also the costs of living in industrial cities are among the main hurdles of foreign students.

“For instance” is not correct here – it does not link smoothly to the previous sentence. In addition, you did mention that Swedish universities were moderately priced, so this needs to be clarified.

Cultural and linguistic differences, the intrinsic difficulty of academic education, and the costs of living in developed countries are among the main hurdles of foreign students.  

• "perpetual monitoring"  =  this sentence has too much information.  

I think you wanted to say something like “by monitoring and supporting the student while they are in the host country” but were worried about word length.  How could we avoid having so many ideas in a single sentence?

Conclusion

As a result it has clearly proved to be useful.

This is very weak, compared to your other sentences. The reference is unclear. It would be great if you could have continued the example of the Bangladeshi student throughout the essay, and into the conclusion. A concluding sentence like this might work:

Studying abroad will allow many bright Bangladeshis and others to realize their ambitions and build their countries.

Ali, before posting again, please do your very best to simplify. Check all sentences for correct structure, and check all nouns for articles.

Thanks!

On the one hand, consider a gifted person in his twenties in Bangladesh. Whereby, corrupted social and political systems as well as widespread poverty have created an inefficient educational system. Meanwhile those untapped potential could be unlocked at Sweden universities. They provide moderately priced, yet productive and modern learning methods and materials for nurturing competent people. If such students return to their countries after graduation, they will significantly contribute to build a better society for the next generation.

On the other hand, this process can be negatively affected if people do not return to home country or fail to complete their education. For Instance, cultural and lingual differences, the intrinsic difficulty of academic education , also the costs of living in industrial cities are among the main hurdles of foreign students. Ultimately, the host country's investment will be lost in this case. Fortunately, These issues can be tackled by selecting people based on appropriate criteria, perpetual monitoring, and person-specific assistance.

In conclusion, although schooling foreign students can be futile in some cases, I strongly believe that if the process is designed and supervised  by experts it will play a pivotal role in prosperity of poor countries. As a result it has clearly proved to be useful.