Why there is difference in entry level posts in PSU

You have a very valid query and there are many students puzzled by this anomaly. Let us see the things in a more understandable perspective. PSUs are categorised based on their performance and profit making and revenue delivered to the Govt. Accordingly they have their own structure and regulation for recruitment. There is no centralised recruitment and each of the PSU has an entrance exam followed by an interview. They induct people at different levels. For staff they take from the state itself either through employment exchanges or open advertisements and for officers they take from all India basis. In executive category many of them recruit as simple engineers as well as graduate trainee engineers and for understanding purpose you can assume that graduate trainee engineers are like IAS of PSUs and there are many who reached to the Director and Chairman level in the organisation through this route. Basic qualification for a simple executive post and a graduate trainee engineer is same but what makes difference is the merit. One has to be high in merit to get selected for the graduate trainee engineer. In fact some PSUs time to time take these executives directly from campus interviews in IITs and other good institutions by selecting the best of the lot.

Now coming to the NOC issue. When you join a PSU, generally you have to execute a bond which is a costly affair if you are going to leave the organisation as the PSU had already spent a lot on the multidisciplinary training on you before joining to your duties. So that is one important thing and if one is ready to pay the money as per bond then PSU would not stop them for leaving the organisation. Then there are issues of notice time that sufficient notice is to be given by the employee before leaving otherwise he will lose his salaries for a few last months. So these are the regulations that are to be seen. Hence, what some people do is they apply elsewhere without proper channel as if they are jobless and if selected pay all these bond money etc, forego the salary and leave. But if you apply through proper channel then until your application is forwarded and approved by your administration it will not be valid and you do not get NOC later while leaving. So, there are some procedures and regulations in each of the PSU that we have to adhere to and there is no short cut to violate them and leave one organisation for another.

Because of the inherent differences in the categorisation of PSUs as per the Govt legislature it is not possible to equate the E1, E2, E3... levels in one PSU to levels in the other and there is no one to one correspondence in this matter. We have to live with that situation and choose our path very carefully after examining these rules and regulations.

You are correct. The entry levels are different in different PSUs. But if you see the salaries at the entry-level. there will not be much difference.
The levels of engineers in different PSUs are different. They don't have a common policy. Recruitment policies are different. They conduct their entry tests separately and they interview separately. Some PSUs are recruiting engineers based on their GATE score also.
The government is having a major stake in all PSUs, they don't want to exchange employees from one PSU to another PSU as it is a loss to the management to lose a trained candidate. So they may not be encouraging the candidate to leave their positions easily.
You have to see what are the terms and conditions of your employment. There may be some condition regarding terminating the job from either side. You have to fulfil those conditions and resign only. When you attend the interview for a new position in another PSU, the new recruiter also will not consider as you don't have NOC.
We all know that there is a lot of unemployment problem in the country and people are looking for jobs and once you get a job in a PSU, nobody will hesitate to join. The scope for development in the career is very good and if you join as a management trainee or graduate engineer trainees immediately after your graduation, there is a scope to get promoted to higher levels in the same organisation are very high. So no one thinks about chances in other PSU. You may have to go with the policies and procedures of the organisations.

All PSU are interested to absorb such candidates having their outstanding performance in both written and oral tests apart from their impressive scorings in their qualification examination. The interview board would see their final performance of all such examinations from class ten onwards to their final stage of their engineering examination in case of recruitment of graduate engineers. Needless to say the entire selection procedure is tough prior to placing them for training for at least a year where the aspirants are provided different inputs to enhance their managerial skills. In that stage also they candidates have to show their potentials. Any slackness relating to their performance would cause extension of their training period.
Though the entry level of such executives would vary in terms of their rankings in designation but their pay - scales and perks are more or less the same.
Their promotional avenue would depend upon how far they pick up the departmental jobs apart from analytical abilities to understand the critical areas of their assignments.
Hence no public undertaking would like to spare their executives once they are inducted in the organisation.
Selecting such candidates from one public undertaking to other can be done through the proper channel only when both the organisations are under the umbrella of one minister such as transfer of a steel plant executive to a mining department. In other case, it is somewhat difficult.
The other option would be to apply to other organisation as a fresh candidate ignoring the privilege enjoyed in the past organisation. If the candidate is under the bond period, he would require to pay back the due amount prior to leaving the organisation so as to join the fresh one. He would be required to tender his resignation ignoring all the previous benifits in relation to perks and his experience.